btrfs: remove redundant zero/NULL initializations in btrfs_alloc_root()

We have allocated the root with kzalloc() so all the memory is already
zero initialized, therefore it's redundant to assign 0 and NULL to several
of the root members. Remove all of them except the atomic initializations
since atomic_t is an opaque type and it's not a good practice to assume
its internals.

This slightly reduces the binary size.
With gcc 14.2.0-19 from Debian on x86_64, before this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939404	 162963	  15592	2117959	 205147	fs/btrfs/btrfs.ko

After this change:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1939212	 162963	  15592	2117767	 205087	fs/btrfs/btrfs.ko

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/disk-io.c

@@ -652,20 +652,10 @@ static struct btrfs_root *btrfs_alloc_root(struct btrfs_fs_info *fs_info,
 	if (!root)
 		return NULL;
 
-	memset(&root->root_key, 0, sizeof(root->root_key));
-	memset(&root->root_item, 0, sizeof(root->root_item));
-	memset(&root->defrag_progress, 0, sizeof(root->defrag_progress));
 	root->fs_info = fs_info;
 	root->root_key.objectid = objectid;
-	root->node = NULL;
-	root->commit_root = NULL;
-	root->state = 0;
 	RB_CLEAR_NODE(&root->rb_node);
 
-	btrfs_set_root_last_trans(root, 0);
-	root->free_objectid = 0;
-	root->nr_delalloc_inodes = 0;
-	root->nr_ordered_extents = 0;
 	xa_init(&root->inodes);
 	xa_init(&root->delayed_nodes);
 
@@ -699,10 +689,7 @@ static struct btrfs_root *btrfs_alloc_root(struct btrfs_fs_info *fs_info,
 	refcount_set(&root->refs, 1);
 	atomic_set(&root->snapshot_force_cow, 0);
 	atomic_set(&root->nr_swapfiles, 0);
-	btrfs_set_root_log_transid(root, 0);
 	root->log_transid_committed = -1;
-	btrfs_set_root_last_log_commit(root, 0);
-	root->anon_dev = 0;
 	if (!btrfs_is_testing(fs_info)) {
 		btrfs_extent_io_tree_init(fs_info, &root->dirty_log_pages,
 					  IO_TREE_ROOT_DIRTY_LOG_PAGES);