gfs2: Fix extended attribute readahead optimization

Commit 39b0555f didn't check for a failing bio_add_page in
gfs2_submit_bhs. This could cause I/O requests to get lost, and the
affected buffer heads to stay locked forever.  Fix that by submitting
the current bio and allocating another one when bio_add_page fails.  (It
is guaranteed that we can at least add one page to a bio.)

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