fs/super: fix emergency thaw double-unlock of s_umount

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

fs/super: fix emergency thaw double-unlock of s_umount

do_thaw_all() iterates over all superblocks via __iterate_supers()
with SUPER_ITER_EXCL, which acquires s_umount exclusively before
calling the callback and releases it afterwards. However, the
callback do_thaw_all_callback() calls thaw_super_locked() which
unconditionally releases s_umount on every code path. This results
in a second unlock attempt in __iterate_supers() that corrupts the
rwsem state, triggering a DEBUG_RWSEMS warning:

[ 182.601148] sysrq: Emergency Thaw of all frozen filesystems
[ 182.601865] ------------[ cut here ]------------
[ 182.602375] DEBUG_RWSEMS_WARN_ON((rwsem_owner(sem) != current) && !rwsem_test_oflags(sem, RWSEM_NONSPINNABLE)): count = 0x0, magic = 0xffff99b1011e5870, owner = 0x0, curr 0xffff99b101b06c80, list not empty
[ 182.603817] WARNING: kernel/locking/rwsem.c:1412 at up_write+0xa3/0x170, CPU#2: kworker/2:1/53
[ 182.604578] Modules linked in:
[ 182.604864] CPU: 2 UID: 0 PID: 53 Comm: kworker/2:1 Not tainted 7.2.0-rc4-00001-gbd3bd93ea98a-dirty #4 PREEMPT(lazy)
[ 182.605711] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.13.0-1kylin1 04/01/2014
[ 182.606417] Workqueue: events do_thaw_all
[ 182.606750] RIP: 0010:up_write+0xaf/0x170
[ 182.607076] Code: 19 3a 92 48 0f 44 c2 48 8b 55 08 48 8b 55 00 4c 8b 45 08 48 8b 55 00 48 8d 3d ad 91 e0 01 48 8b 4d 20 50 48 c7 c6 f0 8c 26 92 <67> 48 0f b9 3a e8 d7 93 4e 00 58 eb 81 48 83 7f 18 00 48 c7 c2 8d
[ 182.608563] RSP: 0018:ffffb670001d7e08 EFLAGS: 00010246
[ 182.609007] RAX: ffffffff92349e8d RBX: 0000000000000000 RCX: ffff99b1011e5870
[ 182.609595] RDX: 0000000000000000 RSI: ffffffff92268cf0 RDI: ffffffff92914d10
[ 182.610283] RBP: ffff99b1011e5870 R08: 0000000000000000 R09: ffff99b101b06c80
[ 182.610847] R10: ffff99b10139a808 R11: fefefefefefefeff R12: 0000000000000000
[ 182.611414] R13: ffffffff90cf74d0 R14: 0000000000000000 R15: ffff99b1011e5800
[ 182.612009] FS: 0000000000000000(0000) GS:ffff99b1eaaee000(0000) knlGS:0000000000000000
[ 182.612670] CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 182.613146] CR2: 00000000005c631c CR3: 00000000013ee000 CR4: 00000000000006f0
[ 182.613722] Call Trace:
[ 182.613946] <TASK>
[ 182.614130] __iterate_supers+0x128/0x150
[ 182.614463] do_thaw_all+0x1b/0x30
[ 182.614759] process_scheduled_works+0xbb/0x3f0
[ 182.615150] ? __pfx_worker_thread+0x10/0x10
[ 182.615499] worker_thread+0x129/0x270
[ 182.615816] ? __pfx_worker_thread+0x10/0x10
[ 182.616201] kthread+0xe2/0x120
[ 182.616469] ? __pfx_kthread+0x10/0x10
[ 182.616792] ret_from_fork+0x15b/0x240
[ 182.617115] ? __pfx_kthread+0x10/0x10
[ 182.617426] ret_from_fork_asm+0x1a/0x30
[ 182.617761] </TASK>
[ 182.617968] ---[ end trace 0000000000000000 ]---
[ 182.618412] Emergency Thaw complete

Fix this by switching to SUPER_ITER_UNLOCKED and acquiring s_umount
in the callback via super_lock_excl() before calling
thaw_super_locked(). This matches the locking pattern expected by
thaw_super_locked() and eliminates the double unlock.

While at it, remove the dead 'return;' at the end of
do_thaw_all_callback().