btrfs: fix empty_zero_page misusage

Heiko Carstens noticed that btrfs was using empty_zero_page
incorrectly.  He explained:

	The definition of empty_zero_page is architecture specific.  It
	is (currently) either a character array, an unsigned long
	containing the address of the empty_zero_page, or even worse
	only the address of the struct page belonging to the
	empty_zero_page.

This commit changes btrfs to use a for-loop instead.  On x86
the resulting .ko is smaller, and we're no longer worrying about
how each arch builds its zeros.
Reported-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>

fs/btrfs/ioctl.c

@@ -368,8 +368,13 @@ static noinline int btrfs_ioctl_fitrim(struct file *file, void __user *arg)
 
 int btrfs_is_empty_uuid(u8 *uuid)
 {
-	BUILD_BUG_ON(BTRFS_UUID_SIZE > PAGE_SIZE);
-	return !memcmp(uuid, empty_zero_page, BTRFS_UUID_SIZE);
+	int i;
+
+	for (i = 0; i < BTRFS_UUID_SIZE; i++) {
+		if (uuid[i])
+			return 0;
+	}
+	return 1;
 }
 
 static noinline int create_subvol(struct inode *dir,