ext4: fix possible UAF when remounting r/o a mmp-protected file system

mainline inclusion
from mainline-5.14
commit	61bb4a1c417e5b95d9edb4f887f131de32e419cb
category: bugfix
bugzilla: 173880
CVE: NA

-------------------------------------------------

After commit 618f003199c6 ("ext4: fix memory leak in
ext4_fill_super"), after the file system is remounted read-only, there
is a race where the kmmpd thread can exit, causing sbi->s_mmp_tsk to
point at freed memory, which the call to ext4_stop_mmpd() can trip
over.

Fix this by only allowing kmmpd() to exit when it is stopped via
ext4_stop_mmpd().

Link: https://lore.kernel.org/r/20210707002433.3719773-1-tytso@mit.edu


Reported-by: Ye Bin <yebin10@huawei.com>
Bug-Report-Link: <20210629143603.2166962-1-yebin10@huawei.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Jan Kara <jack@suse.cz>

Conflicts:
	fs/ext4/super.c

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>