btrfs: remove unused variable in btrfs_{start,write}_dirty_block_groups()

Clang's version of -Wunused-but-set-variable recently gained support for
unary operations, which reveals two unused variables:

  fs/btrfs/block-group.c:2949:6: error: variable 'num_started' set but not used [-Werror,-Wunused-but-set-variable]
          int num_started = 0;
              ^
  fs/btrfs/block-group.c:3116:6: error: variable 'num_started' set but not used [-Werror,-Wunused-but-set-variable]
          int num_started = 0;
              ^
  2 errors generated.

These variables appear to be unused from their introduction, so just
remove them to silence the warnings.

Fixes: c9dc4c6 ("Btrfs: two stage dirty block group writeout")
Fixes: 1bbc621 ("Btrfs: allow block group cache writeout outside critical section in commit")
CC: stable@vger.kernel.org # 5.4+
Link: ClangBuiltLinux#1614
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: nathanchance

fs/btrfs/block-group.c

@@ -2946,7 +2946,6 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans)
 	struct btrfs_path *path = NULL;
 	LIST_HEAD(dirty);
 	struct list_head *io = &cur_trans->io_bgs;
-	int num_started = 0;
 	int loops = 0;
 
 	spin_lock(&cur_trans->dirty_bgs_lock);
@@ -3012,7 +3011,6 @@ int btrfs_start_dirty_block_groups(struct btrfs_trans_handle *trans)
 			cache->io_ctl.inode = NULL;
 			ret = btrfs_write_out_cache(trans, cache, path);
 			if (ret == 0 && cache->io_ctl.inode) {
-				num_started++;
 				should_put = 0;
 
 				/*
@@ -3113,7 +3111,6 @@ int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans)
 	int should_put;
 	struct btrfs_path *path;
 	struct list_head *io = &cur_trans->io_bgs;
-	int num_started = 0;
 
 	path = btrfs_alloc_path();
 	if (!path)
@@ -3171,7 +3168,6 @@ int btrfs_write_dirty_block_groups(struct btrfs_trans_handle *trans)
 			cache->io_ctl.inode = NULL;
 			ret = btrfs_write_out_cache(trans, cache, path);
 			if (ret == 0 && cache->io_ctl.inode) {
-				num_started++;
 				should_put = 0;
 				list_add_tail(&cache->io_list, io);
 			} else {