gfs2: Add GLF_PENDING_REPLY flag

Introduce a new GLF_PENDING_REPLY flag to indicate that a reply from DLM
is expected.  Include that flag in glock dumps to show more clearly
what's going on.  (When the GLF_PENDING_REPLY flag is set, the GLF_LOCK
flag will also be set but the GLF_LOCK flag alone isn't sufficient to
tell that we are waiting for a DLM reply.)

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