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

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>
(cherry picked from commit e6af2ffc86af17c4be427384467c4122fc5301b3)

Change-Id: I1b10a6428b1975ef5c76e1ff40217758ff7d90d9
Reviewed-on: http://gerrit.openafs.org/10248
Reviewed-by: Stephan Wiesand <stephan.wiesand@desy.de>
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Jason Edgecombe <jason@rampaginggeek.com>
Tested-by: BuildBot <buildbot@rampaginggeek.com>