NFS: Show most mount options via nfs_show_options()

Display all mount options in /proc/mount which may be needed to reconstruct
a previous mount.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>