nfsd: clean up nfsd4_close_open_stateid

Minor cleanup that should introduce no behavioral changes.

Currently this function just unhashes the stateid and leaves the caller
to do the work of the CLOSE processing.

Change nfsd4_close_open_stateid so that it handles doing all of the work
of closing a stateid. Move the handling of the unhashed stateid into it
instead of doing that work in nfsd4_close. This will help isolate some
coming changes to stateid handling from nfsd4_close.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>