kern/mm: Use %x and cast for displaying sizeof()

kern/mm: Use %x and cast for displaying sizeof()

There is some variance in how compiler treats sizeof() especially
on 32-bit platforms where it can be naturally either int or long.
Explicit cast solves the issue.

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>