rxrpc: Fix key/keyring checks in setsockopt(RXRPC_SECURITY_KEY/KEYRING)

rxrpc: Fix key/keyring checks in setsockopt(RXRPC_SECURITY_KEY/KEYRING)

An AF_RXRPC socket can be both client and server at the same time.  When
sending new calls (ie. it's acting as a client), it uses rx->key to set the
security, and when accepting incoming calls (ie. it's acting as a server),
it uses rx->securities.

setsockopt(RXRPC_SECURITY_KEY) sets rx->key to point to an rxrpc-type key
and setsockopt(RXRPC_SECURITY_KEYRING) sets rx->securities to point to a
keyring of rxrpc_s-type keys.

Now, it should be possible to use both rx->key and rx->securities on the
same socket - but for userspace AF_RXRPC sockets rxrpc_setsockopt()
prevents that.

Fix this by:

 (1) Remove the incorrect check rxrpc_setsockopt(RXRPC_SECURITY_KEYRING)
     makes on rx->key.

 (2) Move the check that rxrpc_setsockopt(RXRPC_SECURITY_KEY) makes on
     rx->key down into rxrpc_request_key().

 (3) Remove rxrpc_request_key()'s check on rx->securities.

This (in combination with a previous patch) pushes the checks down into the
functions that set those pointers and removes the cross-checks that prevent
both key and keyring being set.

Fixes: 17926a79320a ("[AF_RXRPC]: Provide secure RxRPC sockets for use by userspace and kernel both")
Closes: https://sashiko.dev/#/patchset/20260401105614.1696001-10-dhowells@redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Anderson Nascimento <anderson@allelesecurity.com>
cc: Luxiao Xu <rakukuip@gmail.com>
cc: Yuan Tan <yuantan098@gmail.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260408121252.2249051-16-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>