bitstring: fix ff_area() when start!=0

commit 84e2ae64c597000a0 introduced an error in the ff*_area_at
functions for nonzero start parameters when the bit range sought was
found immediately. It mistakenly replaced '_value = _start' with
'_value = 0' in initialization. Undo that mistake.

Reported by: markj
Reviewed by: markj
Tested by: markj
Fixes: 84e2ae64c597 vm_reserv: use enhanced bitstring for popmaps
Differential Revision: https://reviews.freebsd.org/D35157