DEBUG: sink: add name hint for memory area used by memory-backed sinks

DEBUG: sink: add name hint for memory area used by memory-backed sinks

Thanks to ("MINOR: tools: add vma_set_name() helper"), set a name hint
for user created memory-backed sinks (ring sections without backing-file)
so that they can be easily indentified in /proc/<pid>/maps.

Depending on malloc() implementation, such memory areas will normally be
merged on the heap under MMAP_THRESHOLD (128 kB by default) and will
have a dedicated memory area once the threshold is exceeded. As such, when
large enough, they will appear like this in /proc/<pid>/maps:

7b8e8ac00000-7b8e8bf13000 rw-p 00000000 00:00 0                          [anon:ring:myring]