btrfs: Change the hole range to a more accurate value.

Commit 3ac0d7b96a268a98bd474cab8bce3a9f125aaccf fixed the btrfs expanding
write problem but the hole punched is sometimes too large for some
iovec, which has unmapped data ranges.
This patch will change to hole range to a more accurate value using the
counts checked by the write check routines.

Reported-by: Al Viro <viro@ZenIV.linux.org.uk>
Signed-off-by: Qu Wenruo <quwenruo@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/file.c

@@ -1783,7 +1783,7 @@ static ssize_t btrfs_file_aio_write(struct kiocb *iocb,
 	start_pos = round_down(pos, root->sectorsize);
 	if (start_pos > i_size_read(inode)) {
 		/* Expand hole size to cover write data, preventing empty gap */
-		end_pos = round_up(pos + iov->iov_len, root->sectorsize);
+		end_pos = round_up(pos + count, root->sectorsize);
 		err = btrfs_cont_expand(inode, i_size_read(inode), end_pos);
 		if (err) {
 			mutex_unlock(&inode->i_mutex);