afs: Split the usage count on struct afs_server

Split the usage count on the afs_server struct to have an active count that
registers who's actually using it separately from the reference count on
the object.

This allows a future patch to dispatch polling probes without advancing the
"unuse" time into the future each time we emit a probe, which would
otherwise prevent unused server records from expiring.

Included in this:

 (1) The latter part of afs_destroy_server() in which the RCU destruction
     of afs_server objects is invoked and the outstanding server count is
     decremented is split out into __afs_put_server().

 (2) afs_put_server() now calls __afs_put_server() rather then setting the
     management timer.

 (3) The calls begun by afs_fs_give_up_all_callbacks() and
     afs_fs_get_capabilities() can now take a ref on the server record, so
     afs_destroy_server() can just drop its ref and needn't wait for the
     completion of these calls.  They'll put the ref when they're done.

 (4) Because of (3), afs_fs_probe_done() no longer needs to wake up
     afs_destroy_server() with server->probe_outstanding.

 (5) afs_gc_servers can be simplified.  It only needs to check if
     server->active is 0 rather than playing games with the refcount.

 (6) afs_manage_servers() can propose a server for gc if usage == 0 rather
     than if ref == 1.  The gc is effected by (5).

Signed-off-by: David Howells <dhowells@redhat.com>