selinux: fix selinux_xfrm_alloc_user() to set correct ctx_len

We should count the terminating NUL byte as part of the ctx_len.
Otherwise, UBSAN logs a warning:
  UBSAN: array-index-out-of-bounds in security/selinux/xfrm.c:99:14
  index 60 is out of range for type 'char [*]'

The allocation itself is correct so there is no actual out of bounds
indexing, just a warning.

Cc: [email protected]
Suggested-by: Christian Göttsche <[email protected]>
Link: https://lore.kernel.org/selinux/CAEjxPJ6tA5+LxsGfOJokzdPeRomBHjKLBVR6zbrg+_w3ZZbM3A@mail.gmail.com/
Signed-off-by: Stephen Smalley <[email protected]>
Signed-off-by: Paul Moore <[email protected]>

Author: stephensmalley

security/selinux/xfrm.c

@@ -94,7 +94,7 @@ static int selinux_xfrm_alloc_user(struct xfrm_sec_ctx **ctxp,
 
 	ctx->ctx_doi = XFRM_SC_DOI_LSM;
 	ctx->ctx_alg = XFRM_SC_ALG_SELINUX;
-	ctx->ctx_len = str_len;
+	ctx->ctx_len = str_len + 1;
 	memcpy(ctx->ctx_str, &uctx[1], str_len);
 	ctx->ctx_str[str_len] = '\0';
 	rc = security_context_to_sid(ctx->ctx_str, str_len,