LinuxKPI: add seq_hex_dump()

Move the implementation of print_hex_dump() into linux_compat.c as
lkpi_hex_dump() taking an extra function pointer and argument.
Add two internal wrappers for printf and sbuf_printf to get a common
function definition (sbuf_printf takes the extra argument).

Use these to implement print_hex_dump() and the newly added
seq_hex_dump().

This allows us to re-use the same implementation of print_hex_dump()
for both functions without duplicating the code.

Initial implementation: D49381 by dumbbell
Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D49637