MFC r305202: MFV r302654:
6879 incorrect endianness swap for drr_spill.drr_length in libzfs_sendrecv.c

illumos/illumos-gate@20fea7a47472aceb64d3ed48cc2a3ea268bc4795
https://github.com/illumos/illumos-gate/commit/20fea7a47472aceb64d3ed48cc2a3ea26
8bc4795

https://www.illumos.org/issues/6879

In libzfs_sendrecv, there's a typo:
case DRR_SPILL:
            if (byteswap) {
                drr->drr_u.drr_write.drr_length =
                    BSWAP_64(drr->drr_u.drr_spill.drr_length);
            }
Instead of drr_write.drr_length, we should be assigning the result of the
byteswap to drr_spill.drr_length.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Dan Kimmel <dan.kimmel@delphix.com>