linuxkpi: Implement <linux/seq_buf.h>

It is a wrapper above a char * to track the overall available space in
the buffer as well as the used space. This wrapper does not manage
memory allocation.

The DRM generic code started to use this in Linux 6.10.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D54488