BUILD: sink: replace S_IRUSR, S_IWUSR with their octal value

BUILD: sink: replace S_IRUSR, S_IWUSR with their octal value

The build broke on freebsd with S_IRUSR undefined after commit 0b8e9ceb1
("MINOR: ring: add support for a backing-file"). Maybe another include
is needed there, but the point is that we really don't care about these
symbolic names, file modes are more readable as 0600 than via these
cryptic names anyway, so let's go back to 0600. This will teach me not
to try to make things too clean.

No backport is needed.