nfsd: fix an IS_ERR() vs NULL check

The get_backchannel_cred() used to return error pointers on error but
now it returns NULL pointers.

Fixes: 97f68c6b ("SUNRPC: add 'struct cred *' to auth_cred and rpc_cre")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>