Btrfs-progs: rework open_ctree to take flags, add a new one

So I needed to add a flag to not try to read block groups when doing
--init-extent-tree since we could hang there, but that meant adding a whole
other 0/1 type flag to open_ctree_fs_info.  So instead I've converted it all
over to using a flags setting and added the flag that I needed.  This has been
tested with xfstests and make test.  Thanks,

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