linux: provide read and write ops even when we have aio

linux: provide read and write ops even when we have aio

read/write will fall back to aio ops but e.g. writev will
fail if there is not either a write or writev op explicitly.

force the fallback via do_sync_read/do_sync_write

required with 2.6.18-348.x rhel kernels but probably not newer ones

Change-Id: I773a8e38df435015e4bc9fc353d930d14b3e6791
Reviewed-on: http://gerrit.openafs.org/10246
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@your-file-system.com>
Tested-by: Derrick Brashear <shadow@your-file-system.com>