btrfs: fix comment about nbytes increase at replay_one_extent()

The comment is wrong about the part where it says a prealloc extent does
not contribute to an inode's nbytes - it does. Only holes don't contribute
and that's what we are checking for, as prealloc extents always have a
disk_bytenr different from 0. So fix the comment and re-organize the code
to not set nbytes twice and set it to the extent item's number of bytes
only if it doesn't represent a hole - in case it's a hole we have already
initialized nbytes to 0 when we declared it.

Reviewed-by: Qu Wenruo <wqu@suse.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/tree-log.c

@@ -740,15 +740,10 @@ static noinline int replay_one_extent(struct walk_control *wc)
 
 	if (found_type == BTRFS_FILE_EXTENT_REG ||
 	    found_type == BTRFS_FILE_EXTENT_PREALLOC) {
-		nbytes = btrfs_file_extent_num_bytes(wc->log_leaf, item);
-		extent_end = start + nbytes;
-
-		/*
-		 * We don't add to the inodes nbytes if we are prealloc or a
-		 * hole.
-		 */
-		if (btrfs_file_extent_disk_bytenr(wc->log_leaf, item) == 0)
-			nbytes = 0;
+		extent_end = start + btrfs_file_extent_num_bytes(wc->log_leaf, item);
+		/* Holes don't take up space. */
+		if (btrfs_file_extent_disk_bytenr(wc->log_leaf, item) != 0)
+			nbytes = btrfs_file_extent_num_bytes(wc->log_leaf, item);
 	} else if (found_type == BTRFS_FILE_EXTENT_INLINE) {
 		size = btrfs_file_extent_ram_bytes(wc->log_leaf, item);
 		nbytes = btrfs_file_extent_ram_bytes(wc->log_leaf, item);