LinuxKPI: mm.h: implement virt_to_head_page()

Implement virt_to_head_page().

Sponsored by:	The FreeBSD Foundation
MFC after:	3 days
Reviewed by:	markj
Differential Revision: https://reviews.freebsd.org/D38240

sys/compat/linuxkpi/common/include/linux/mm.h

@@ -41,6 +41,7 @@
 #include <linux/list.h>
 #include <linux/mmap_lock.h>
 #include <linux/shrinker.h>
+#include <linux/page.h>
 
 #include <asm/pgtable.h>
 
@@ -151,6 +152,13 @@ struct sysinfo {
 	uint32_t mem_unit;	/* Memory unit size in bytes */
 };
 
+static inline struct page *
+virt_to_head_page(const void *p)
+{
+
+	return (virt_to_page(p));
+}
+
 /*
  * Compute log2 of the power of two rounded up count of pages
  * needed for size bytes.