LinuxKPI: add virt_to_phys()

Add virt_to_phys() as a define to vtophys().
This is used by a wireless driver for dma related work; sigh.

MFC after:	3 days
Reviewed by:	emaste
Differential Revision: https://reviews.freebsd.org/D37301

sys/compat/linuxkpi/common/include/asm/io.h

@@ -31,6 +31,13 @@
 #ifndef _LINUXKPI_ASM_IO_H_
 #define _LINUXKPI_ASM_IO_H_
 
+#include <sys/param.h>
+
+#include <vm/vm.h>
+#include <vm/pmap.h>
+
 #include <linux/io.h>
 
+#define	virt_to_phys(x)		vtophys(x)
+
 #endif	/* _LINUXKPI_ASM_IO_H_ */