linuxkpi: Fix atomic_long_sub() overflow

By (ab)using atomic_long_add_return(), atomic_long_sub() was making
the atomic long overflow. Indeed the underlying FreeBSD atomic is based
on an unsigned long.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D38090

(cherry picked from commit 9491ea7c68221ca7bc5e369ebb57660886ef1b13)