Linux 4.14 compat: CONFIG_GCC_PLUGIN_RANDSTRUCT

Fix build errors with gcc 7.2.0 on Gentoo with kernel 4.14
built with CONFIG_GCC_PLUGIN_RANDSTRUCT=y such as:

module/nvpair/nvpair.c:2810:2:error:
positional initialization of field in ?struct? declared with
'designated_init' attribute [-Werror=designated-init]

  nvs_native_nvlist,
  ^~~~~~~~~~~~~~~~~

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Mark Wright <gienah@gentoo.org>
Closes #5390
Closes #6903