ext4: use sb_rdonly() helper for checking read-only flag

sb_rdonly() helper instead of directly checking sb->s_flags.

Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20230616165109.21695-2-jack@suse.cz


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