rxrpc: Fix checker warning

Fix the following checker warning:

../net/rxrpc/key.c:692:9: error: subtraction of different types can't work (different address spaces)

Checker is wrong in this case, but cast the pointers to unsigned long to
avoid the warning.

Whilst we're at it, reduce the assertions to WARN_ON() and return an error.

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