NFSv4: Ensure nfs_atomic_open set the dentry verifier on ENOENT

commit 809fd143de8805970eec02c27c0bc2622a6ecbda upstream.

If the OPEN rpc call to the server fails with an ENOENT call, nfs_atomic_open
will create a negative dentry for that file, however it currently fails
to call nfs_set_verifier(), thus causing the dentry to be immediately
revalidated on the next call to nfs_lookup_revalidate() instead of following
the usual lookup caching rules.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>