BUG/MINOR: sink: make sure to always properly unmap a file-backed ring

BUG/MINOR: sink: make sure to always properly unmap a file-backed ring

The munmap() call performed on exit was incorrect since it used to apply
to the buffer instead of the area, so neither the pointer nor the size
were page-aligned. This patches corrects this and also adds a call to
msync() since munmap() alone doesn't guarantee that data will be dumped.

This should be backported to 2.6.