linuxkpi: Update posittion after copy in seq_read()

seq_read() is usually called in a loop because the destination buffer
might be smaller than the source. The caller relies on the updated
position to read what is next.

We also use memcpy() instead of strscpy() because we don't need to
append a NUL character.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51560