Btrfs-progs: add --init-extent-tree to btrfsck

Btrfs-progs: add --init-extent-tree to btrfsck

In some cases the extent tree can just be so gone there is no point in trying to
figure out how to put it back together.  So add a --init-extent-tree mode which
will zero out the extent tree and then re-add extents for all of the blocks we
find.  This will also undo any balance that was going on at the time of the
crash, this is needed because the reloc tree seems to confuse fsck at the
moment.  With this patch I can put back together a users file system that was
completely gone.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>