Fix a logic bug in the FreeBSD getpages VOP

In commit cd32b4f5b79c ("Fix a deadlock in the FreeBSD getpages VOP") I
introduced a bug while porting the patch originally committed to
FreeBSD: the rangelock pointer may be NULL if the try operation failed,
so we must avoid calling zfs_rangelock_unlock() in that case.

Reviewed-by: Allan Jude <allan@klarasystems.com>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Reviewed-by: Matt Macy <mmacy@FreeBSD.org>
Reported-by: Steve Wills <swills@FreeBSD.org>
Signed-off-by: Mark Johnston <markj@FreeBSD.org>
Closes #10519
Closes #10960