When OSD_OP_RETURN_CHAIN is set, the primary OSD appends chain info to
the returned bitmap buffer. The space required for chain info is
(len / bs_bitmap_granularity) * key_index_bytes, where key_index_bytes
depends on the snapshot chain depth (1 byte for <= 255 items, 2 bytes for
<= 65535, etc.).
Previously, cur_op->reply.rw.bitmap_len only added (len / bs_bitmap_granularity)
without multiplying by key_index_bytes. When the snapshot chain depth
exceeded 255 (key_index_bytes == 2), the reported bitmap_len was too short,
causing the client to receive truncated chain info.
Multiply by key_index_bytes when updating cur_op->reply.rw.bitmap_len, and
add test_chained_read_large_chain_info regression test in osd_test.
