Allow realpath to work for file mounts

For file mounts, the directory vnode is not available from namei and this
prevents the use of vn_fullpath_hardlink. In this case, we can use the
vnode which was covered by the file mount with vn_fullpath.

This also disallows file mounts over files with link counts greater than
one to ensure a deterministic path to the mount point.

Reviewed by: mjg, kib
Tested by: pho