MFC r337185:
MFV r337184: 9457 libzfs_import.c:add_config() has a memory leak

A memory leak occurs on lines 209 and 213 because the config is not freed
in the error case. The interface to add_config() seems less than ideal -
it would be better if it copied any data necessary from the config and the
caller freed it.

illumos/illumos-gate@ddfe901b12348d31c500fb57f9174e88860a4061

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim.dimitro@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: sara hartse <sara.hartse@delphix.com>