btrfs: factor out reading of bg from find_frist_block_group

When find_first_block_group() finds a block group item in the extent-tree,
it does a lookup of the object in the extent mapping tree and does further
checks on the item.

Factor out this step from find_first_block_group() so we can further
simplify the code.

While we're at it, we can also just return early in
find_first_block_group(), if the tree slot isn't found.

Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>