afs: Combine the endpoint state bools into a bitmask

Combine the endpoint state bool-type members into a bitmask so that some of
them can be waited upon more easily.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org