btrfs: zoned: fix wrong use of bitops API in btrfs_ensure_empty_zones
find_next_bit and find_next_zero_bit take @size as the second parameter and
@offset as the third parameter. They are specified opposite in
btrfs_ensure_empty_zones(). Thanks to the later loop, it never failed to
detect the empty zones. Fix them and (maybe) return the result a bit
faster.
Note: the naming is a bit confusing, size has two meanings here, bitmap
and our range size.