Test case misc/021 is testing if we could mount a single disk btrfs
image recovered from multi disk fs.

The problem is, current kernel has extra check for block group, chunk
and dev extent.
This means any image can't pass btrfs check for chunk tree will not
mount.

So do extra btrfs check before mount, this will also help us to locate
the problem in btrfs-image easier.

Signed-off-by: Qu Wenruo <wqu@suse.com>