!11163 btrfs: get rid of warning on transaction commit when using flushoncommit

static inline int btrfs_start_delalloc_flush(struct btrfs_fs_info *fs_info)

{

	/*

	 * We use writeback_inodes_sb here because if we used

	 * We use try_to_writeback_inodes_sb() here because if we used

	 * btrfs_start_delalloc_roots we would deadlock with fs freeze.

	 * Currently are holding the fs freeze lock, if we do an async flush

	 * we'll do btrfs_join_transaction() and deadlock because we need to

	 * wait for the fs freeze lock.  Using the direct flushing we benefit

	 * from already being in a transaction and our join_transaction doesn't

	 * have to re-take the fs freeze lock.

	 *

	 * Note that try_to_writeback_inodes_sb() will only trigger writeback

	 * if it can read lock sb->s_umount. It will always be able to lock it,

	 * except when the filesystem is being unmounted or being frozen, but in

	 * those cases sync_filesystem() is called, which results in calling

	 * writeback_inodes_sb() while holding a write lock on sb->s_umount.

	 * Note that we don't call writeback_inodes_sb() directly, because it

	 * will emit a warning if sb->s_umount is not locked.

	 */

	if (btrfs_test_opt(fs_info, FLUSHONCOMMIT))

		writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);

		try_to_writeback_inodes_sb(fs_info->sb, WB_REASON_SYNC);

	return 0;

}