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.

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>
(cherry picked from commit e5ba359f64b5d8d874a31a50694398c17f27cfa1)

Change-Id: I8d8ee81c2bcff2dc85c2ff3e70452a0af5639921
Reviewed-on: http://gerrit.openafs.org/9493
Reviewed-by: Andrew Deason <adeason@sinenomine.net>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Reviewed-by: Michael Meffie <mmeffie@sinenomine.net>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>