zfsctl_snapshot_vptocnp(): Use vn_busy_mountedhere()

[olce]

Summary

This is an impact of the protocol change to act on the mount point(s) covering
a given vnode and in particular crossing them.

This commit is (hopefully) the last in a series whose goal is to remove the
VV_CROSSLOCK vnode flag and logic and fix a timing-dependent deadlock between
lookup and unmount.

[olce]

Update the prediction predicate after vn_busy_mountedhere()'s change of contract.