gfs2: Clean up {lookup,fillup}_metapath

Split out the entire lookup loop from lookup_metapath and
fillup_metapath.  Make both functions return the actual height in
mp->mp_aheight, and return 0 on success.  Handle lookup errors properly
in trunc_dealloc.

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