Fix for crash on MIPS /dev/mem read.

Reading /dev/mem in 64 bit kernel crashes.  This is because the page
used to call uiomove_fromphys() from memrw() does not have md.pv_list
initialized correctly.

The fix is to call pmap_page_init() on the page to initialize it.

sys/mips/mips/mem.c

@@ -87,6 +87,7 @@ memrw(struct cdev *dev, struct uio *uio, int flags)
 
 	GIANT_REQUIRED;
 
+	pmap_page_init(&m);
 	while (uio->uio_resid > 0 && !error) {
 		iov = uio->uio_iov;
 		if (iov->iov_len == 0) {