Don't create a dedicated session for each AIO kernel process.

[jhb]

Summary

Don't create a dedicated session for each AIO kernel process.

This code dates back to the initial AIO support and the commit log does
not explain why it is needed. However, I cannot find anything in the
AIO code or the various file methods (fo_read/fo_write) that would change
behavior due to using a private session instead of proc0's session.

Test Plan

• Ran all of the aio tests in the test suite. No regressions.

[kib]

I think what could be different with the call is the handling of the SIGTTYIN/SIGTTYOUT conditions for terminal i/o. But I believe that aio daemons groups cannot be in background set for any terminal, regardless of whether they are in proc0 or its own group.