rxrpc: Fix UAF in rxgk_issue_challenge()

Fix rxgk_issue_challenge() to free the page containing the challenge
content after invoking the tracepoint as the whdr passed to the tracepoint
points into the page just freed.

Fixes: 9d1d2b59341f ("rxrpc: rxgk: Implement the yfs-rxgk security class (GSSAPI)")
Reported-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: stable@kernel.org
Link: https://patch.msgid.link/20260609140911.838677-4-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>