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

commit 5694c93e6c4954fa9424c215f75eeb919bddad64 upstream.

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>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
[bwh: Backported to 3.2:
 - Adjust context
 - Also move initialisation of nfs4_client::cl_strhash, but
   not nfs4_client::cl_revoked]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>