linuxkpi: seq_read: Fix off by one error

strscpy needs the buffer length not the string length (so including
the '\0').

Reviewed by: bz
Fixes: f697b9432d9c ("linuxkpi: drm-kmod debugfs support")
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D37771