ext4: return error on syncfs after shutdown

This is the logic behavior and one that we would like to verify
using a generic fstest similar to xfs/546.

Link: https://lore.kernel.org/fstests/20240830152648.GE6216@frogsfrogsfrogs/


Suggested-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Amir Goldstein <amir73il@gmail.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240904084657.1062243-1-amir73il@gmail.com


Signed-off-by: Theodore Ts'o <tytso@mit.edu>