vfs: add a work around for vp_crossmp bug to realpath

The actual bug is not yet addressed as it will get much easier after other
problems are addressed (most notably rename contract).

The only affected in-tree consumer is realpath. Everyone else happens to be
performing lookups within a mount point, having a side effect of ni_dvp being
set to mount point's root vnode in the worst case.

Reported by: pho