Split gethrtime() and gethrtime_waitfree() and make the former use

Split gethrtime() and gethrtime_waitfree() and make the former use
nanouptime() instead of getnanouptime().  nanouptime(9) provides more
precise result at expense of being slower.

In r269223, gethrtime() is used as creation time of dbuf, which in turn
acts as portion of lookup key to maintain AVL invariant where there can
not be duplicate items.  Before this change, gethrtime() have preferred
better execution time by sacrificing precision, which may lead to panic
on busy systems with:

	panic: avl_find() succeeded inside avl_add()

Reported by:	allanjude, mav
PR:		kern/192284
MFC after:	11 days
X-MFC-with:	r269223