Page fault handling fix.

Factored out mapping of the physical page as the final generic code
after all fault-specific handling is done.

Fixed the error that zero page didn't have an owner (devzero).

Fixed the error that struct dirent did not have the record length
field as u16 as expected by userspace.

tasks/test0/src/dirtest.c

@@ -104,9 +104,9 @@ void print_dirents(char *path, void *buf, int cnt)
 
 int lsdir(char *path)
 {
-	int fd;
 	struct dirent dents[DENTS_TOTAL];
 	int bytes;
+	int fd;
 
 	memset(dents, 0, sizeof(struct dirent) * DENTS_TOTAL);