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

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/[email protected]
Reported-by: Ye Bin <[email protected]>
Bug-Report-Link: <[email protected]>
Signed-off-by: Theodore Ts'o <[email protected]>
Reviewed-by: Jan Kara <[email protected]>

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