UPSTREAM: gfs2: Clean up function may_grant

commit 61444649 upstream

Pass the first current glock holder into function may_grant and
deobfuscate the logic there.

While at it, switch from BUG_ON to GLOCK_BUG_ON in may_grant.  To make
that build cleanly, de-constify the may_grant arguments.

We're now using function find_first_holder in do_promote, so move the
function's definition above do_promote.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
(cherry picked from commit b25cfbc0)
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: Ib02a752cdf8da470494fa06782bc92ca3f047a6f