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

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