gfs2: Get rid of GLF_INVALIDATE_IN_PROGRESS

Get rid of the GLF_INVALIDATE_IN_PROGRESS flag: it was originally used
to indicate to add_to_queue() that the ->go_sync() and ->go_invalid()
operations were in progress, but as we have established in commit "gfs2:
Fix LM_FLAG_TRY* logic in add_to_queue", add_to_queue() has no need to
know.

Commit d99724c3 describes a race in which GLF_INVALIDATE_IN_PROGRESS
is used to serialize two processes which are both in do_xmote() at the
same time.  That analysis is wrong: the serialization happens via the
GLF_LOCK flag, which ensures that at most one glock operation can be
active at any time.

Fixes: d99724c3 ("gfs2: Close timing window with GLF_INVALIDATE_IN_PROGRESS")
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Reviewed-by: Andrew Price <anprice@redhat.com>