nfs: make nfs_path() work without vfsmount

part 3: now we have everything to get nfs_path() just by dentry -
just follow to (disconnected) root and pick the rest of the thing
there.

Start killing propagation of struct vfsmount * on the paths that
used to bring it to nfs_path().

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>