linuxkpi: Add hex_dump_to_buffer()

This function prints a single line of hex dump to the given line buffer.

The implementation relies on lkpi_hex_dump() to format the string.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D51559

(cherry picked from commit b1bef9f7e0e182a9eee222639ffc8af4883ec0fa)