This was used to make sure that a fresh btrfs from an older mkfs.btrfs,
but it also allows us to mount a buggy btrfs if this btrfs has the right
superblock head part but has something wrong with chunk tree part[1], and
after that we can hit BUG_ON()s set in the code to prevent something
impossible.

Since David has released "Btrfs progs v3.19-rc2", just remove the check,
if anyone who wants to make a fresh btrfs, please use the latest one.

[1]: http://www.spinics.net/lists/linux-btrfs/msg42358.htmlSigned-off-by: NLiu Bo <bo.li.liu@oracle.com>
Reviewed-by: NOmar Sandoval <osandov@osandov.com>
Reviewed-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <clm@fb.com>