NDFREE(): Fix unlocking for LOCKPARENT|LOCKLEAF and ndp->ni_dvp == ndp->ni_vp.

NDFREE() calculates unlock_dvp after ndp->ni_vp is unlocked and zeroed
out. This makes the comparision of ni_dvp with ni_vp always fail.
Move the calculation of unlock_dvp right after unlock_vp, so that the
code sees correct ni_vp value.

Reproduced by

	   chdir("/usr");
	   open("/..", O_BENEATH | O_RDONLY);

Reported by: syzkaller
Reviewed by: markj, mckusick
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D20304