nfsd: Use the filehandle to look up the struct nfs4_file instead of inode

This makes more sense anyway since an inode pointer value can change
even when the filehandle doesn't.

Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Jeff Layton <jlayton@primarydata.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>