btrfs: handle error in btrfs_cache_block_group

We have a BUG_ON(ret < 0) in find_free_extent from
btrfs_cache_block_group.  If we fail to allocate our ctl we'll just
panic, which is not good.  Instead just go on to another block group.
If we fail to find a block group we don't want to return ENOSPC, because
really we got a ENOMEM and that's the root of the problem.  Save our
return from btrfs_cache_block_group(), and then if we still fail to make
our allocation return that ret so we get the right error back.

Tested with inject-error.py from bcc.

Reviewed-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>