ring: Consistently use size_t to specify ring size

The mm_len member of struct ring is of type size_t, but in the code
paths leading to set it, unsigned int is used. In circumstances where
unsigned int is 32 bit and size_t is 64 bit, this could lead to an
integer overflow, which causes an improper ring size being mmap()'ed in
mmap_ring_generic().

In order to prevent this, consistently use size_t to store the ring
size, since this is also what mmap() takes as its `length' parameter.

This now allows to specify ring sizes larger than 4 GiB for both
netsniff-ng and trafgen (fixes #90).

Reported-by: Jon Schipp <jonschipp@gmail.com>
Reported-by: Michał Purzyński <michalpurzynski1@gmail.com>
Signed-off-by: Tobias Klauser <tklauser@distanz.ch>