xfs: widen timestamps to deal with y2038

This series performs some refactoring of our timestamp and inode
encoding functions, then retrofits the timestamp union to handle
timestamps as a 64-bit nanosecond counter.  Next, it refactors the quota
grace period expiration timer code a bit before implementing bit
shifting to widen the effective counter size to 34 bits.  This enables
correct time handling on XFS through the year 2486.