ext4: Rename ext4_free_blks_{count,set}() to refer to clusters

The field bg_free_blocks_count_{lo,high} in the block group
descriptor has been repurposed to hold the number of free clusters for
bigalloc functions.  So rename the functions so it makes it easier to
read and audit the block allocation and block freeing code.

Note: at this point in bigalloc development we doesn't support
online resize, so this also makes it really obvious all of the places
we need to fix up to add support for online resize.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
diff --git a/fs/ext4/ext4.h b/fs/ext4/ext4.h
index a5a7e36..db279c3 100644
--- a/fs/ext4/ext4.h
+++ b/fs/ext4/ext4.h
@@ -1978,8 +1978,8 @@
 				      struct ext4_group_desc *bg);
 extern ext4_fsblk_t ext4_inode_table(struct super_block *sb,
 				     struct ext4_group_desc *bg);
-extern __u32 ext4_free_blks_count(struct super_block *sb,
-				struct ext4_group_desc *bg);
+extern __u32 ext4_free_group_clusters(struct super_block *sb,
+				      struct ext4_group_desc *bg);
 extern __u32 ext4_free_inodes_count(struct super_block *sb,
 				 struct ext4_group_desc *bg);
 extern __u32 ext4_used_dirs_count(struct super_block *sb,
@@ -1992,8 +1992,9 @@
 				  struct ext4_group_desc *bg, ext4_fsblk_t blk);
 extern void ext4_inode_table_set(struct super_block *sb,
 				 struct ext4_group_desc *bg, ext4_fsblk_t blk);
-extern void ext4_free_blks_set(struct super_block *sb,
-			       struct ext4_group_desc *bg, __u32 count);
+extern void ext4_free_group_clusters_set(struct super_block *sb,
+					 struct ext4_group_desc *bg,
+					 __u32 count);
 extern void ext4_free_inodes_set(struct super_block *sb,
 				struct ext4_group_desc *bg, __u32 count);
 extern void ext4_used_dirs_set(struct super_block *sb,