linuxkpi: Add krealloc_array()

In FreeBSD, this is a wrapper on top of realloc().

V2: Check if n * size would overflow and return NULL if that's the

case. Suggested by hselasky@ and emaste@.

Reviewed by: manu
Approved by: manu
Differential Revision: https://reviews.freebsd.org/D36959

(cherry picked from commit 1ad6b2b1daa8937b2e1ced43802adba5734ba92b)