btrfs: decrease indentation of find_free_extent_update_loop

btrfs: decrease indentation of find_free_extent_update_loop

Decrease the indentation of find_free_extent_update_loop(), by inverting
the check if the loop state is smaller than LOOP_NO_EMPTY_SIZE.

This also allows for an early return from find_free_extent_update_loop(),
in case LOOP_NO_EMPTY_SIZE is already set at this point.

While at it change a

    if () {
    }
    else if
    else

pattern to all using curly braces and be consistent with the rest of btrfs
code.

Also change 'int exists' to 'bool have_trans' giving it a more meaningful
name and type.

No functional changes intended.

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