trace: rename trace_print_hex_seq arg and add kdoc

Steven suggested to improve trace_print_hex_seq() a bit after commit
2acae0d5

 ("trace: add variant without spacing in trace_print_hex_seq")
in two ways: i) by adding a kdoc comment for the helper function
itself and ii) by renaming 'spacing' argument into 'concatenate'
to better denote that we don't add spaces between each hex bytes.

Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>