SUNRPC: Zero rpc_gss_wire_cred at svcauth_gss_decode_credbody() entry

svcauth_gss_decode_credbody() writes the caller's
rpc_gss_wire_cred field by field and assigns gc_ctx.len only on
the success tail.  The caller storage is svcdata->clcred, which
lives in the per-svc_rqst gss_svc_data and is reused across
requests.  Early decode failures leave partially decoded state
mixed with residue from the prior request.

The trailing body_len tightness check is the sharpest case:
xdr_stream_decode_opaque_inline() has already written gc_ctx.data
with a borrowed inline pointer into the current request's XDR
pages, but gc_ctx.len retains its prior value.  Once the request
pages are released the pooled clcred carries a dangling pointer
paired with a stale length.

Zero the caller's rpc_gss_wire_cred at function entry so that
every early-return path leaves a deterministic all-zero cred.
On the trailing tightness-check path, gc_ctx.len is now zero
instead of stale, which neuters length-driven consumers such as
gss_svc_searchbyctx() that would otherwise walk the dangling
data pointer.

Fixes: b0bc53470d ("SUNRPC: Convert the svcauth_gss_accept() pre-amble to use xdr_stream")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <clm@meta.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Link: https://patch.msgid.link/20260528-tier2-v1-5-d026a1415e0b@oracle.com
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/auth_gss/svcauth_gss.c

@@ -1575,6 +1575,9 @@ svcauth_gss_decode_credbody(struct xdr_stream *xdr,
 	u32 body_len;
 	__be32 *p;
 
+	/* Early-return paths leave deterministic state, not stale residue. */
+	memset(gc, 0, sizeof(*gc));
+
 	p = xdr_inline_decode(xdr, XDR_UNIT);
 	if (!p)
 		return false;