linuxkpi: add strncpy_from_user()

[denis.borovikov_gmail.com]

Summary

Same semantics as Linux: returns the string length without the NUL, count if the string was truncated, or -EFAULT. Implemented with copyinstr(9). Unlike linux_copyin() it does not translate the remapped ioctl argument window; its user, drm/virtio, passes a pointer taken from the ioctl payload, so that is not needed.

Test Plan

Built on main with this applied: linuxkpi, linuxkpi_wlan, mlx5, iwlwifi and ath10k for aarch64 and amd64, and drm-kmod's virtio-gpu (freebsd/drm-kmod#517) without its interim copy of this function.

[denis.borovikov_gmail.com]

Used by freebsd/drm-kmod#517

[wulf]

sys/compat/linuxkpi/common/include/linux/uaccess.h

38

#include <sys/systm.h> should immediately follow #include <sys/param.h>. See man 9 style

[denis.borovikov_gmail.com]

sys/compat/linuxkpi/common/include/linux/uaccess.h

38

done

[denis.borovikov_gmail.com]

<sys/systm.h> now follows <sys/param.h>, as wulf asked; nothing else.