Fix NanoBSD when the data partition size is defined as a negative

Fix NanoBSD when the data partition size is defined as a negative
number.

	It is possible to ask nanobsd.sh to create a 'data' partition,
	separate from the system or configuration partitions, and
	furthermore, by specifying a negative value for its size
	to request that it use all space unused by those partitions
	for its own size.

	Because the two lines of code that calculate how much space
	is available for this data partition are written in perl-like
	syntax, the awk code that does the processing performs the
	calculation incorrectly.
	[note - this was already fixed by r174936]

	Furthermore, a comparison later down fails to newfs the
	partition when the size is negative.

PR:		misc/127759
Submitted by:	Cyrus Rahman <crahman@gmail.com>
MFC after:	1 week