Btrfs: fix 64 bit divide problem

This fixes a regression introduced by commit cdcb725c05fe ("Btrfs: check
if there is enough space for balancing smarter").  We can't do 64-bit
divides on 32-bit architectures.

In cases where we need to divide/multiply by 2 we should just left/right
shift respectively, and in cases where theres N number of devices use
do_div.  Also make the counters u64 to match up with rw_devices.
Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Acked-and-tested-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/btrfs/extent-tree.c

@@ -6735,9 +6735,9 @@ int btrfs_can_relocate(struct btrfs_root *root, u64 bytenr)
 	struct btrfs_fs_devices *fs_devices = root->fs_info->fs_devices;
 	struct btrfs_device *device;
 	u64 min_free;
+	u64 dev_min = 1;
+	u64 dev_nr = 0;
 	int index;
-	int dev_nr = 0;
-	int dev_min = 1;
 	int full = 0;
 	int ret = 0;
 
@@ -6796,14 +6796,16 @@ int btrfs_can_relocate(struct btrfs_root *root, u64 bytenr)
 	index = get_block_group_index(block_group);
 	if (index == 0) {
 		dev_min = 4;
-		min_free /= 2;
+		/* Divide by 2 */
+		min_free >>= 1;
 	} else if (index == 1) {
 		dev_min = 2;
 	} else if (index == 2) {
-		min_free *= 2;
+		/* Multiply by 2 */
+		min_free <<= 1;
 	} else if (index == 3) {
 		dev_min = fs_devices->rw_devices;
-		min_free /= dev_min;
+		do_div(min_free, dev_min);
 	}
 
 	mutex_lock(&root->fs_info->chunk_mutex);