Handle truncation while processing a file

[darius-dons.net.au]

Summary

If the file is truncated during processing the loop will spin forever.

See https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=196694

Test Plan

I put a break point in the loop and truncated the file, previously it would spin forever, with the change it doesn't.

[ngie]

Based on internal testing done and the fact that this change is shipping in current versions of OneFS, I say, ShipIt!

[delphij]

Looks good to me, the code should take read(2) returning 0 as a signal for breaking the loop.

[ngie]

Committed as r281928 -- thank you!