dlm: fix plock invalid read

In the Linux kernel, the following vulnerability has been resolved:

dlm: fix plock invalid read

This patch fixes an invalid read showed by KASAN. A unlock will allocate a
"struct plock_op" and a followed send_op() will append it to a global
send_list data structure. In some cases a followed dev_read() moves it
to recv_list and dev_write() will cast it to "struct plock_xop" and access
fields which are only available in those structures. At this point an
invalid read happens by accessing those fields.

To fix this issue the "callback" field is moved to "struct plock_op" to
indicate that a cast to "plock_xop" is allowed and does the additional
"plock_xop" handling if set.

Example of the KASAN output which showed the invalid read:

[ 2064.296453] ==================================================================
[ 2064.304852] BUG: KASAN: slab-out-of-bounds in dev_write+0x52b/0x5a0 [dlm]
[ 2064.306491] Read of size 8 at addr ffff88800ef227d8 by task dlm_controld/7484
[ 2064.308168]
[ 2064.308575] CPU: 0 PID: 7484 Comm: dlm_controld Kdump: loaded Not tainted 5.14.0+ #9
[ 2064.310292] Hardware name: Red Hat KVM, BIOS 0.5.1 01/01/2011
[ 2064.311618] Call Trace:
[ 2064.312218] dump_stack_lvl+0x56/0x7b
[ 2064.313150] print_address_description.constprop.8+0x21/0x150
[ 2064.314578] ? dev_write+0x52b/0x5a0 [dlm]
[ 2064.315610] ? dev_write+0x52b/0x5a0 [dlm]
[ 2064.316595] kasan_report.cold.14+0x7f/0x11b
[ 2064.317674] ? dev_write+0x52b/0x5a0 [dlm]
[ 2064.318687] dev_write+0x52b/0x5a0 [dlm]
[ 2064.319629] ? dev_read+0x4a0/0x4a0 [dlm]
[ 2064.320713] ? bpf_lsm_kernfs_init_security+0x10/0x10
[ 2064.321926] vfs_write+0x17e/0x930
[ 2064.322769] ? __fget_light+0x1aa/0x220
[ 2064.323753] ksys_write+0xf1/0x1c0
[ 2064.324548] ? __ia32_sys_read+0xb0/0xb0
[ 2064.325464] do_syscall_64+0x3a/0x80
[ 2064.326387] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 2064.327606] RIP: 0033:0x7f807e4ba96f
[ 2064.328470] Code: 89 54 24 18 48 89 74 24 10 89 7c 24 08 e8 39 87 f8 ff 48 8b 54 24 18 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 01 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 31 44 89 c7 48 89 44 24 08 e8 7c 87 f8 ff 48
[ 2064.332902] RSP: 002b:00007ffd50cfe6e0 EFLAGS: 00000293 ORIG_RAX: 0000000000000001
[ 2064.334658] RAX: ffffffffffffffda RBX: 000055cc3886eb30 RCX: 00007f807e4ba96f
[ 2064.336275] RDX: 0000000000000040 RSI: 00007ffd50cfe7e0 RDI: 0000000000000010
[ 2064.337980] RBP: 00007ffd50cfe7e0 R08: 0000000000000000 R09: 0000000000000001
[ 2064.339560] R10: 000055cc3886eb30 R11: 0000000000000293 R12: 000055cc3886eb80
[ 2064.341237] R13: 000055cc3886eb00 R14: 000055cc3886f590 R15: 0000000000000001
[ 2064.342857]
[ 2064.343226] Allocated by task 12438:
[ 2064.344057] kasan_save_stack+0x1c/0x40
[ 2064.345079] __kasan_kmalloc+0x84/0xa0
[ 2064.345933] kmem_cache_alloc_trace+0x13b/0x220
[ 2064.346953] dlm_posix_unlock+0xec/0x720 [dlm]
[ 2064.348811] do_lock_file_wait.part.32+0xca/0x1d0
[ 2064.351070] fcntl_setlk+0x281/0xbc0
[ 2064.352879] do_fcntl+0x5e4/0xfe0
[ 2064.354657] __x64_sys_fcntl+0x11f/0x170
[ 2064.356550] do_syscall_64+0x3a/0x80
[ 2064.358259] entry_SYSCALL_64_after_hwframe+0x44/0xae
[ 2064.360745]
[ 2064.361511] Last potentially related work creation:
[ 2064.363957] kasan_save_stack+0x1c/0x40
[ 2064.365811] __kasan_record_aux_stack+0xaf/0xc0
[ 2064.368100] call_rcu+0x11b/0xf70
[ 2064.369785] dlm_process_incoming_buffer+0x47d/0xfd0 [dlm]
[ 2064.372404] receive_from_sock+0x290/0x770 [dlm]
[ 2064.374607] process_recv_sockets+0x32/0x40 [dlm]
[ 2064.377290] process_one_work+0x9a8/0x16e0
[ 2064.379357] worker_thread+0x87/0xbf0
[ 2064.381188] kthread+0x3ac/0x490
[ 2064.383460] ret_from_fork+0x22/0x30
[ 2064.385588]
[ 2064.386518] Second to last potentially related work creation:
[ 2064.389219] kasan_save_stack+0x1c/0x40
[ 2064.391043] __kasan_record_aux_stack+0xaf/0xc0
[ 2064.393303] call_rcu+0x11b/0xf70
[ 2064.394885] dlm_process_incoming_buffer+0x47d/0xfd0 [dlm]
[ 2064.397694] receive_from_sock+0x290/0x770
---truncated---