gfs2: return errors from gfs2_ail_empty_gl

Before this patch, function gfs2_ail_empty_gl did not return errors it
encountered from __gfs2_trans_begin. Those errors usually came from the
fact that the file system was made read-only, often due to unmount
(but theoretically could be due to -o remount,ro), which prevented
the transaction from starting.

The inability to start a transaction prevented its revokes from being
properly written to the journal for glocks during unmount (and
transition to ro).

That meant glocks could be unlocked without the metadata properly
revoked in the journal. So other nodes could grab the glock thinking
that their lvb values were correct, but in fact corresponded to the
glock without its revokes properly synced. That presented as lvb
mismatch errors.

This patch allows gfs2_ail_empty_gl to return the error properly to
the caller.

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