Simplify the test against maxproc in fork1().

Previously nprocs_new would be tested against maxprocs twice when
nprocs_new < maxprocs - 10. Eliminate the unnecessary comparison.

Submitted by: Wuyang Chung <wuyang.chung1@gmail.com>
GitHub PR: https://github.com/freebsd/freebsd/pull/397
MFC after: 1 week