LinuxKPI: implement krealloc() for memory being contiguous

Implement krealloc_array() using krealloc(). Implement krealloc()
doing the various size checks ourselves and use realloc() or kmalloc()
depending on old and new allocation sizes.
This way we can ensure that allocated memory stays physically contiguous.

Sponsored by: The FreeBSD Foundation
Suggested by: jhb (see D46657)
Reviewed by: jhb, markj
Differential Revision: https://reviews.freebsd.org/D49571

(cherry picked from commit 1c95d401ebe5075ebb38b57638830713a496f107)