btrfs: trim adjacent extents across adjacent block groups boundary

btrfs: trim adjacent extents across adjacent block groups boundary

The test case checks if btrfs can trim adjacent extents across
adjacent block groups boundary correctly.

The test case craft the following extents layout:

         |  BG1 |      BG2        |       BG3            |
 Bytenr: X-8M   X      X+512M     X+1G  X+1G+128M
         |//////|//////|          |     |//|

There is a long existing bug that, when btrfs is trying to trim the
range at [X+512M, X+1G+128M), it will only trim [X+512M, X+1G).

This test case is the regression test for this long existing bug.

It will verify the trimmed bytes by using loopback device backed up
by a file, and checking the bytes used by the file to determine how
many bytes are trimmed.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>