gfs2: split glock instantiation off from do_promote

Before this patch, function do_promote had a section of code that did
the actual instantiation.  This patch splits that off into its own
function, gfs2_instantiate, which prepares us for the next patch that
will use that function.

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