net: pktgen: fix mix of int/long

Fix mix of int/long (and multiple conversion from/to) by using consequently
size_t for i and max and ssize_t for len and adjust function signatures
of hex32_arg(), count_trail_chars(), num_arg() and strn_len() accordingly.

Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>