ext4_handle_error() with errors=continue mount option can accidentally
remount the filesystem read-only when the system is rebooting. Fix that.

Fixes: 1dc1097f ("ext4: avoid panic during forced reboot")
Signed-off-by: Jan Kara <jack@suse.cz>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/20201127113405.26867-2-jack@suse.czSigned-off-by: Theodore Ts'o <tytso@mit.edu>