zfsctl_mounted_here(): Use vn_cross_single_mount()

[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 part of 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]

Fix a bug where '*vpp' was set to NULL even when EJUSTRETURN is returned. Fix
probable build warnings on old versions.