Btrfs: use linux/sizes.h to represent constants

We use many constants to represent size and offset value.  And to make
code readable we use '256 * 1024 * 1024' instead of '268435456' to
represent '256MB'.  However we can make far more readable with 'SZ_256MB'
which is defined in the 'linux/sizes.h'.

So this patch replaces 'xxx * 1024 * 1024' kind of expression with
single 'SZ_xxxMB' if 'xxx' is a power of 2 then 'xxx * SZ_1M' if 'xxx' is
not a power of 2. And I haven't touched to '4096' & '8192' because it's
more intuitive than 'SZ_4KB' & 'SZ_8KB'.

Signed-off-by: Byongho Lee <bhlee.kernel@gmail.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/ctree.h

@@ -35,6 +35,7 @@
 #include <linux/btrfs.h>
 #include <linux/workqueue.h>
 #include <linux/security.h>
+#include <linux/sizes.h>
 #include "extent_io.h"
 #include "extent_map.h"
 #include "async-thread.h"
@@ -196,9 +197,9 @@ static int btrfs_csum_sizes[] = { 4 };
 /* ioprio of readahead is set to idle */
 #define BTRFS_IOPRIO_READA (IOPRIO_PRIO_VALUE(IOPRIO_CLASS_IDLE, 0))
 
-#define BTRFS_DIRTY_METADATA_THRESH	(32 * 1024 * 1024)
+#define BTRFS_DIRTY_METADATA_THRESH	SZ_32M
 
-#define BTRFS_MAX_EXTENT_SIZE (128 * 1024 * 1024)
+#define BTRFS_MAX_EXTENT_SIZE SZ_128M
 
 /*
  * The key defines the order in the tree, and so it also defines (optimal)