Add support for optional cgroup attributes

Cgroup attributes like io.weight or io.bfq.weight only exist if the
corresponding I/O scheduler has been loaded. Hence add support for
ignoring failures to write to a cgroup attribute that does not exist.

Bug: 213617178
Test: This patch has been tested in combination with patch "Migrate the blkio controller to the cgroup v2 hierarchy".
Change-Id: I5debafabc4dd430a1b98ea343bf3740c764edcc3
Signed-off-by: Bart Van Assche <bvanassche@google.com>