Btrfs: add pinned extents to on-disk free space cache correctly

I got this while running xfstests:

[24256.836098] block group 317849600 has an wrong amount of free space
[24256.836100] btrfs: failed to load free space cache for block group 317849600

We should clamp the extent returned by find_first_extent_bit(),
so the start of the extent won't smaller than the start of the
block group.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>