mm/slub: Move krealloc() and related code to slub.c

This is a preparation for the following refactoring of krealloc(),
for more efficient function calling as it will call some internal
functions defined in slub.c.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Vlastimil Babka <vbabka@suse.cz>