In the root fs search use the correct unit number when checking for

In the root fs search use the correct unit number when checking for
device existinance.  The new device infrastructure often registers disk
unist separately, so looking up unit 0 when only unit N exists will not
succeed.  We might want to later augment this with the slice and partition
number (which are 0 at the time the search is made prior to the real lookup),
but this should work for now.

Submitted-by: Hiroki Sato <hrs@allbsd.org>