rxrpc: rxgk: Implement the yfs-rxgk security class (GSSAPI)

Implement the basic parts of the yfs-rxgk security class (security index 6)
to support GSSAPI-negotiated security.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250411095303.2316168-9-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>