ext4: replace read-only check for shutdown check in mmp code

ext4: replace read-only check for shutdown check in mmp code

The multi-mount protection kthread checks for read-only filesystem and
aborts in that case. The remount code actually handles stopping of the
kthread on remount so the only purpose of the check is in case of
emergency remount read-only. Replace the check for read-only filesystem
with a check for shutdown filesystem as running MMP on such is risky
anyway and it makes ordering of things during remount simpler.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230616165109.21695-11-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>