btrfs: only account delalloc bytes for regular file inodes in btrfs_getattr()

btrfs_getattr() unconditionally reads BTRFS_I(inode)->new_delalloc_bytes
and adds it (sector-aligned) to stat->blocks for every inode type.
However, new_delalloc_bytes lives in a union with last_dir_index_offset:

    union {
        u64 new_delalloc_bytes;     /* files only */
        u64 last_dir_index_offset;  /* directories only */
    };

For a directory inode this memory holds last_dir_index_offset, which is
set during directory logging (e.g. flush_dir_items_batch()) to the
offset of the last logged BTRFS_DIR_INDEX_KEY.  That offset grows with
the number of entries ever created in the directory (dir indexes are
monotonic and never reused), so it can be arbitrarily large.

As a result, after a directory has been logged (e.g. via an fsync that
triggers directory logging), btrfs_getattr() reports inflated st_blocks
for that directory.  The inflation is purely in-core and disappears
after the inode is evicted and reloaded (btrfs_alloc_inode() zeroes the
union), e.g. after a remount.

Reproducer (on a btrfs filesystem):

    D=/mnt/btrfs/d
    mkdir -p $D
    for i in $(seq 1 20000); do touch $D/f$i; done
    sync                      # commit, push dir index high
    touch $D/trigger          # dirty the dir in a new transaction
    xfs_io -c fsync $D        # log the directory -> sets last_dir_index_offset
    stat -c '%b' $D           # st_blocks is now inflated (e.g. 40)
    # umount + mount -> st_blocks drops back to the correct value

The evict path already knows this union is type-dependent and guards the
corresponding WARN_ON with !S_ISDIR() in btrfs_destroy_inode(); only
btrfs_getattr() was missing the equivalent check.

Only read new_delalloc_bytes for regular files, which are the only
inodes that ever set it.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Dave Chen <davechen@synology.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -8068,7 +8068,8 @@ static int btrfs_getattr(struct mnt_idmap *idmap,
 	stat->result_mask |= STATX_SUBVOL;
 
 	spin_lock(&BTRFS_I(inode)->lock);
-	delalloc_bytes = BTRFS_I(inode)->new_delalloc_bytes;
+	delalloc_bytes = S_ISREG(inode->i_mode) ?
+			 BTRFS_I(inode)->new_delalloc_bytes : 0;
 	inode_bytes = inode_get_bytes(inode);
 	spin_unlock(&BTRFS_I(inode)->lock);
 	stat->blocks = (ALIGN(inode_bytes, blocksize) +