Btrfs: output warning instead of error when loading free space cache failed

Btrfs: output warning instead of error when loading free space cache failed

commit 32d6b47fe6fc1714d5f1bba1b9f38e0ab0ad58a8 upstream.

If we fail to load a free space cache, we can rebuild it from the extent tree,
so it is not a serious error, we should not output a error message that
would make the users uncomfortable. This patch uses warning message instead
of it.

Signed-off-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Chris Mason <clm@fb.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>