Fix: mm_page_alloc_extfrag instrumentation for kernel 3.18.10

Fix: mm_page_alloc_extfrag instrumentation for kernel 3.18.10

The following commit has been backported from kernel 3.19.2 into
3.18.10, introducing a change to mm_page_alloc_extfrag and therefore
provoking a build failure on a 3.18.10 kernel:

commit a4f3f96f850db69890f47984c60606a2362bee89
Author: Vlastimil Babka <vbabka@suse.cz>
Date:   Wed Feb 11 15:28:15 2015 -0800

    mm: when stealing freepages, also take pages created by splitting buddy page

    commit 99592d598eca62bdbbf62b59941c189176dfc614 upstream.

This patch adds the proper version check to allow building on 3.18.y.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>