SUNRPC: Fix an error code in gss_alloc_msg()

If kstrdup_const() then this function returns zero (success) but it
should return -ENOMEM.

Fixes: ac83228a ("SUNRPC: Use namespace of listening daemon in the client AUTH_GSS upcall")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>