ext4: Make checks for metadata_csum feature safer

This is just a small part of commit dec214d0 "ext4: xattr inode
deduplication" that makes checks for metadata_csum feature safer and is
actually needed by following fixes.

Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Acked-by: Jan Kara <jack@suse.cz>
[bwh: Ported to 3.16: Use EXT4_HAS_RO_COMPAT_FEATURE()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>