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

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