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.

Reviewed by: kib
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D4988