btrfs: add a test case to check btrfs won't crash on certain corruption

btrfs: add a test case to check btrfs won't crash on certain corruption

The test case would reproduce the situation by creating an empty fs,
with SINGLE metadata profile, then corrupt the tree root manually.
Finally try mounting the corrupted fs, the mount should fail while our
kernel should not fail.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
[ Update commit log. Fix a line gt 80 chars. Use append to $seqres.full.
  Fix comment ]
Signed-off-by: Anand Jain <anand.jain@oracle.com>