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.

Fixes: 1cd6121f ("btrfs: zoned: implement zoned chunk allocator")
CC: stable@vger.kernel.org # 5.15+
Signed-off-by: Naohiro Aota <naohiro.aota@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>