rpcsec_gss: Fix a stack overflow in svc_rpc_gss_validate()

svc_rpc_gss_validate() copies the input message into a stack buffer
without ensuring that the buffer is large enough. Sure enough,
oa_length may be up to 400 bytes, much larger than the provided space.
This enables an unauthenticated user to trigger an overflow and obtain
remote code execution.

Add a runtime check which verifies that the copy won't overflow.

Approved by: so
Security: FreeBSD-SA-26:08.rpcsec_gss
Security: CVE-2026-4747
Reported by: Nicholas Carlini <npc@anthropic.com>
Reviewed by: rmacklem
Fixes: a9148abd9da5d