rgw: Fix Browser POST content-length-range min value

rgw: Fix Browser POST content-length-range min value

`ERR_TOO_SMALL` is wrongly returned if all of the following are true,
- the get_data returns multiple items (chunks)
- the length of the last item is smaller than the POST Policy's min
  value for content-length-range.

The check should be `(ofs < min_len)` instead of `(len < min_len)`

This is further confirmed by the next line of `s->obj_size = ofs`

Move the `int len` scope inside loop to try and prevent the bug in
future.

The bug was refactored in 2016, but was introduced in Oct 2012, when
this functionality was first added to RGW in commit 7bb3504d3f0974e9863f536e9af0ce8889d6888f.

Reference: https://github.com/ceph/ceph/blob/933a42f9af349b3b222270e7f19f1fe151d89e8e/src/rgw/rgw_op.cc#L4474-L4513
Reference: https://github.com/ceph/ceph/commit/7bb3504d3f0974e9863f536e9af0ce8889d6888f
Signed-off-by: Robin H. Johnson <rjohnson@digitalocean.com>