ext4: teach mballoc preallocation code about bigalloc clusters

In most of mballoc.c, we do everything in units of clusters, since the
block allocation bitmaps and buddy bitmaps are all denominated in
clusters.  The one place where we do deal with absolute block numbers
is in the code that handles the preallocation regions, since in the
case of inode-based preallocation regions, the start of the
preallocation region can't be relative to the beginning of the group.

So this adds a bit of complexity, where pa_pstart and pa_lstart are
block numbers, while pa_free, pa_len, and fe_len are denominated in
units of clusters.
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

fs/ext4/mballoc.h

@@ -139,9 +139,9 @@ enum {
 
 struct ext4_free_extent {
 	ext4_lblk_t fe_logical;
-	ext4_grpblk_t fe_start;
+	ext4_grpblk_t fe_start;	/* In cluster units */
 	ext4_group_t fe_group;
-	ext4_grpblk_t fe_len;
+	ext4_grpblk_t fe_len;	/* In cluster units */
 };
 
 /*