gfs2: Rename handle_callback to request_demote

Function handle_callback() is used to request a glock demote.  This
often happens in response to a conflicting remote locking request and
subsequent bast callback from DLM, but there are other reasons for
triggering a demote request as well, such as when trying to release a
glock in response to memory pressure.  To clarify that, rename the
function to request_demote().

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