NFS: Move declaration of nfs_mount() to fs/nfs/internal.h

Clean up:  The nfs_mount() function is not to be used outside of the
NFS client.  Move its public declaration to fs/nfs/internal.h.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>