Address code review comments

Change INT_MAX to INT32_MAX.
Document iovec == NULL.
Strengthen TODO about re-architecting futex code.
Rename getSize() to size().
Simplify error handling for mFifo.diff().
Rename masked to offset.

Test: re-run tests/*
Change-Id: I51f0c457399ac12b282eee726d13c3ca9a58ed1a
2 files changed