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
MFC after: 3 days
Suggested by: jhb (see D46657)
Reviewed by: jhb, markj
Differential Revision: https://reviews.freebsd.org/D49571