Btrfs-progs: add a free space cache checker to fsck V2

Btrfs-progs: add a free space cache checker to fsck V2

In trying to track down a weird tree log problem I wanted to make sure that the
free space cache was actually valid, which we currently have no way of doing.
So this patch adds a bunch of support for the free space cache code and then a
checker to fsck.  Basically we go through and if we can actually load the free
space cache then we will walk the extent tree and verify that the free space
cache exactly matches what is in the extent tree.  Hopefully this will always be
correct, the only time it wouldn't is if the extent tree is corrupt or we have
some sort of awful bug in the free space cache.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>