LINUX: Detect non-vectorized aio functions

LINUX: Detect non-vectorized aio functions

In kernels before 027445c3, the functions generic_file_aio_read and
generic_file_aio_write, as well as the fs operations aio_read and
aio_write, do not deal with iovecs but rather just use a single
buffer. Detect this, so our aio_read and aio_write implementations
have the correct signatures.

This removes several warnings on such kernels.

Change-Id: I70aa0d43bac5545d83710806b58f36d13d7f6cc8
Reviewed-on: http://gerrit.openafs.org/8020
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>