"Fix spurious sanity check failures in destroy_workqueue()" introduced memory leaks

[Qian Cai]

The linux-next commit [1] introduced a lot of memory leaks while running LTP fs
tests (/opt/ltp/runltp -f fs). Reverted the commit fixed the problem. It seems
the new code wq->rescuer to NULL without free it first, so later rcu_free_wq()
is unable to free it.

unreferenced object 0xffff8f822e7c8428 (size 192):
 comm "fs_fill", pid 8310, jiffies 4294965449 (age 494.720s)
 hex dump (first 32 bytes):
ÂÂÂÂ28 84 7c 2e 82 8f ff ff 28 84 7c 2e 82 8f ff ffÂÂ(.|.....(.|.....
ÂÂÂÂ00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00ÂÂ................
 backtrace:
ÂÂÂÂ[<0000000030795d2f>] kmem_cache_alloc_node_trace+0x153/0x470
ÂÂÂÂ[<000000000398512e>] alloc_worker+0x21/0x50
ÂÂÂÂ[<000000001934cb3a>] init_rescuer.part.4+0x1d/0xb0
ÂÂÂÂ[<00000000f36417e5>] alloc_workqueue+0x336/0x5f3
ÂÂÂÂ[<0000000011066bf3>] ext4_fill_super+0x1eec/0x2f70 [ext4]
ÂÂÂÂ[<000000001fdee6fd>] mount_bdev+0x191/0x1c0
ÂÂÂÂ[<00000000e2d3e265>] ext4_mount+0x15/0x20 [ext4]
ÂÂÂÂ[<00000000631fe1d9>] legacy_get_tree+0x34/0x60
ÂÂÂÂ[<00000000ee947c38>] vfs_get_tree+0x27/0xb0
ÂÂÂÂ[<00000000f4bcb594>] do_mount+0x8a0/0xae0
ÂÂÂÂ[<0000000005dd5056>] ksys_mount+0xb6/0xd0
ÂÂÂÂ[<00000000fb14e10a>] __x64_sys_mount+0x25/0x30
ÂÂÂÂ[<000000009fb1dd3e>] do_syscall_64+0x6d/0x488
ÂÂÂÂ[<00000000a822a6c4>] entry_SYSCALL_64_after_hwframe+0x49/0xbe
unreferenced object 0xffff8f824c314218 (size 192):
 comm "fs_fill", pid 8310, jiffies 4294966011 (age 489.100s)
 hex dump (first 32 bytes):
ÂÂÂÂ18 42 31 4c 82 8f ff ff 18 42 31 4c 82 8f ff ffÂÂ.B1L.....B1L....
ÂÂÂÂ00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00ÂÂ................
 backtrace:
ÂÂÂÂ[<0000000030795d2f>] kmem_cache_alloc_node_trace+0x153/0x470
ÂÂÂÂ[<000000000398512e>] alloc_worker+0x21/0x50
ÂÂÂÂ[<000000001934cb3a>] init_rescuer.part.4+0x1d/0xb0
ÂÂÂÂ[<00000000f36417e5>] alloc_workqueue+0x336/0x5f3
ÂÂÂÂ[<000000000b6d2e88>] xfs_init_mount_workqueues+0x2a/0x150 [xfs]
ÂÂÂÂ[<0000000014c756cd>] xfs_fs_fill_super+0x311/0x760 [xfs]
ÂÂÂÂ[<000000001fdee6fd>] mount_bdev+0x191/0x1c0
ÂÂÂÂ[<00000000d0aa6706>] xfs_fs_mount+0x15/0x20 [xfs]
ÂÂÂÂ[<00000000631fe1d9>] legacy_get_tree+0x34/0x60
ÂÂÂÂ[<00000000ee947c38>] vfs_get_tree+0x27/0xb0
ÂÂÂÂ[<00000000f4bcb594>] do_mount+0x8a0/0xae0
ÂÂÂÂ[<0000000005dd5056>] ksys_mount+0xb6/0xd0
ÂÂÂÂ[<00000000fb14e10a>] __x64_sys_mount+0x25/0x30
ÂÂÂÂ[<000000009fb1dd3e>] do_syscall_64+0x6d/0x488
ÂÂÂÂ[<00000000a822a6c4>] entry_SYSCALL_64_after_hwframe+0x49/0xbe

[1] https://lore.kernel.org/lkml/20190919014340.GM3084169@xxxxxxxxxxxxxxxxxxxxxx
k.com/