NFSd: Move default initialisers from create_client() to alloc_client()

Aside from making it clearer what is non-trivial in create_client(), it
also fixes a bug whereby we can call free_client() before idr_init()
has been called.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Cc: stable@vger.kernel.org
Signed-off-by: J. Bruce Fields <bfields@redhat.com>