fs/jfs: prevent double-free in dbUnmount() after failed jfs_remount()

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

fs/jfs: prevent double-free in dbUnmount() after failed jfs_remount()

Syzkaller reported the following issue:
==================================================================
BUG: KASAN: double-free in slab_free mm/slub.c:3787 [inline]
BUG: KASAN: double-free in __kmem_cache_free+0x71/0x110 mm/slub.c:3800
Free of addr ffff888086408000 by task syz-executor.4/12750
[...]
Call Trace:
<TASK>
[...]
kasan_report_invalid_free+0xac/0xd0 mm/kasan/report.c:482
____kasan_slab_free+0xfb/0x120
kasan_slab_free include/linux/kasan.h:177 [inline]
slab_free_hook mm/slub.c:1781 [inline]
slab_free_freelist_hook+0x12e/0x1a0 mm/slub.c:1807
slab_free mm/slub.c:3787 [inline]
__kmem_cache_free+0x71/0x110 mm/slub.c:3800
dbUnmount+0xf4/0x110 fs/jfs/jfs_dmap.c:264
jfs_umount+0x248/0x3b0 fs/jfs/jfs_umount.c:87
jfs_put_super+0x86/0x190 fs/jfs/super.c:194
generic_shutdown_super+0x130/0x310 fs/super.c:492
kill_block_super+0x79/0xd0 fs/super.c:1386
deactivate_locked_super+0xa7/0xf0 fs/super.c:332
cleanup_mnt+0x494/0x520 fs/namespace.c:1291
task_work_run+0x243/0x300 kernel/task_work.c:179
resume_user_mode_work include/linux/resume_user_mode.h:49 [inline]
exit_to_user_mode_loop+0x124/0x150 kernel/entry/common.c:171
exit_to_user_mode_prepare+0xb2/0x140 kernel/entry/common.c:203
__syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
syscall_exit_to_user_mode+0x26/0x60 kernel/entry/common.c:296
do_syscall_64+0x49/0xb0 arch/x86/entry/common.c:86
entry_SYSCALL_64_after_hwframe+0x63/0xcd
[...]
</TASK>

Allocated by task 13352:
kasan_save_stack mm/kasan/common.c:45 [inline]
kasan_set_track+0x3d/0x60 mm/kasan/common.c:52
____kasan_kmalloc mm/kasan/common.c:371 [inline]
__kasan_kmalloc+0x97/0xb0 mm/kasan/common.c:380
kmalloc include/linux/slab.h:580 [inline]
dbMount+0x54/0x980 fs/jfs/jfs_dmap.c:164
jfs_mount+0x1dd/0x830 fs/jfs/jfs_mount.c:121
jfs_fill_super+0x590/0xc50 fs/jfs/super.c:556
mount_bdev+0x26c/0x3a0 fs/super.c:1359
legacy_get_tree+0xea/0x180 fs/fs_context.c:610
vfs_get_tree+0x88/0x270 fs/super.c:1489
do_new_mount+0x289/0xad0 fs/namespace.c:3145
do_mount fs/namespace.c:3488 [inline]
__do_sys_mount fs/namespace.c:3697 [inline]
__se_sys_mount+0x2d3/0x3c0 fs/namespace.c:3674
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Freed by task 13352:
kasan_save_stack mm/kasan/common.c:45 [inline]
kasan_set_track+0x3d/0x60 mm/kasan/common.c:52
kasan_save_free_info+0x27/0x40 mm/kasan/generic.c:518
____kasan_slab_free+0xd6/0x120 mm/kasan/common.c:236
kasan_slab_free include/linux/kasan.h:177 [inline]
slab_free_hook mm/slub.c:1781 [inline]
slab_free_freelist_hook+0x12e/0x1a0 mm/slub.c:1807
slab_free mm/slub.c:3787 [inline]
__kmem_cache_free+0x71/0x110 mm/slub.c:3800
dbUnmount+0xf4/0x110 fs/jfs/jfs_dmap.c:264
jfs_mount_rw+0x545/0x740 fs/jfs/jfs_mount.c:247
jfs_remount+0x3db/0x710 fs/jfs/super.c:454
reconfigure_super+0x3bc/0x7b0 fs/super.c:935
vfs_fsconfig_locked fs/fsopen.c:254 [inline]
__do_sys_fsconfig fs/fsopen.c:439 [inline]
__se_sys_fsconfig+0xad5/0x1060 fs/fsopen.c:314
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x3d/0xb0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x63/0xcd
[...]

JFS_SBI(ipbmap->i_sb)->bmap wasn't set to NULL after kfree() in
dbUnmount().

Syzkaller uses faultinject to reproduce this KASAN double-free
warning. The issue is triggered if either diMount() or dbMount() fail
in jfs_remount(), since diUnmount() or dbUnmount() already happened in
such a case - they will do double-free on next execution: jfs_umount
or jfs_remount.

Tested on both upstream and jfs-next by syzkaller.