LU-12198 libcfs: always copy ioctl header back to user

LU-12198 libcfs: always copy ioctl header back to user

lnetctl_get_peer_list fills back the required size in header if the
given buffer was too small. Userspace needs the info back to grow
the buffer and try again.

Note we only replace err on failure if err was previously not set

Lustre-change: https://review.whamcloud.com/37559
Lustre-commit: 9e02ef474f8caa833d6a1b5e0068d5323a57e8c4

Fixes: fba98579efc4 ("LU-6202 libcfs: replace libcfs_register_ioctl with a blocking notifier_chain")
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>
Change-Id: I2b6e319aceeb00d488572053d27023891afe1928
Reviewed-by: Yang Sheng <ys@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Neil Brown <neilb@suse.de>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/37720
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>