mem: make page_is_zero externally linked

We'll use it in anon shmem dedup so we need to have access
to it in shmem.c
Signed-off-by: Eugene Batalov <eabatalov89@gmail.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/include/mem.h

@@ -17,6 +17,7 @@ struct mem_dump_ctl {
 };
 
 extern bool vma_has_guard_gap_hidden(struct vma_area *vma);
+extern bool page_is_zero(u64 pme);
 extern bool page_in_parent(bool dirty);
 extern int prepare_mm_pid(struct pstree_item *i);
 extern void prepare_cow_vmas(void);

criu/mem.c

@@ -115,7 +115,7 @@ bool should_dump_page(VmaEntry *vmae, u64 pme)
 	return false;
 }
 
-static inline bool page_is_zero(u64 pme)
+bool page_is_zero(u64 pme)
 {
 	return (pme & PME_PFRAME_MASK) == kdat.zero_page_pfn;
 }