NFS: Move the flock open mode check into nfs_flock()

We only need to check lock exclusive/shared types against open mode when
flock() is used on NFS, so move it into the flock-specific path instead of
checking it for all locks.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>