gfs2: Extended attribute readahead optimization

Instead of submitting a READ_SYNC bio for the inode and a READA bio for
the inode's extended attributes through submit_bh, submit a single READ_SYNC
bio for both through submit_bio when possible.  This can be more
efficient on some kinds of block devices.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>