OpenZFS 6879 - Incorrect endianness swap

OpenZFS 6879 - Incorrect endianness swap

Authored by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Ported-by: Denys Rtveliashvili <denys@rtveliashvili.name>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Incorrect endianness swap for drr_spill.drr_length in libzfs_sendrecv.c
Instead of drr_write.drr_length, we should be assigning the result of
the byteswap to drr_spill.drr_length.

OpenZFS-issue: https ://www.illumos.org/issues/6879
OpenZFS-commit: https ://github.com/openzfs/openzfs/commit/74c8720
Closes #4613