rxrpc: Fix key parsing memleak

rxrpc: Fix key parsing memleak

In rxrpc_preparse_xdr_yfs_rxgk(), the memory attached to token->rxgk can be
leaked in a few error paths after it's allocated.

Fix this by freeing it in the "reject_token:" case.

Fixes: 0ca100ff4df6 ("rxrpc: Add YFS RxGK (GSSAPI) security class")
Closes: https://sashiko.dev/#/patchset/20260319150150.4189381-1-dhowells%40redhat.com
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Jeffrey Altman <jaltman@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260408121252.2249051-3-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>