This adds support for synchronous iocbs and converts generic_file_read
to use a sync iocb to call into generic_file_aio_read.

The tests I've run with lmbench on a piii-866 showed no difference in
file re-read speed when forced to use a completion path via aio_complete
and an -EIOCBQUEUED return from generic_file_aio_read -- people with
slower machines might want to test this to see if we can tune it any
better.  Also, a bug fix to correct a missing call into the aio code
from the fork code is present.  This patch sets things up for making
generic_file_aio_read actually asynchronous.