mm/slub: remove freelist_dereference()

freelist_dereference() is a one-liner only used from get_freepointer().
Remove it and make get_freepointer() call freelist_ptr_decode()
directly to make the code easier to follow.

Signed-off-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Kees Cook <keescook@chromium.org>