mm: memcg/slab: fix slab statistics in !SMP configuration

Since commit ea426c2a

 ("mm: memcg: prepare for byte-sized vmstat
items") the write side of slab counters accepts a value in bytes and
converts it to pages.  It happens in __mod_node_page_state().

However a non-SMP version of __mod_node_page_state() doesn't perform
this conversion.  It leads to incorrect (unrealistically high) slab
counters values.  Fix this by adding a similar conversion to the non-SMP
version of __mod_node_page_state().

Signed-off-by: Roman Gushchin <guro@fb.com>
Reported-and-tested-by: Bastian Bittorf <bb@npl.de>
Fixes: ea426c2a

 ("mm: memcg: prepare for byte-sized vmstat items")
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>