gfs2: Fix remote demote of weak glock holders

When we mock up a temporary holder in gfs2_glock_cb to demote weak holders in
response to a remote locking conflict, we don't set the HIF_HOLDER flag.  This
causes function may_grant to BUG.  Fix by setting the missing HIF_HOLDER flag
in the mock glock holder.

In addition, define the mock glock holder where it is used.

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