gfs2: Clean up function may_grant

gfs2: Clean up function may_grant

BugLink: https://bugs.launchpad.net/bugs/1976135
commit 6144464937fe1e6135b13a30502a339d549bf093 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>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>