SUNRPC: Fix up an error return value in gss_import_sec_context_kerberos()

If the context allocation fails, the function currently returns a random
error code, since the variable 'p' still points to a valid memory location.

Ensure that it returns ENOMEM...

Cc: stable@kernel.org
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>