Linux: cleanup aio support

Linux: cleanup aio support

Code that called directly into the aio operations (ex: readv/writev)
would bypass the AFS specific operations found in afs_linux_read
and afs_linux_write.

Rework the handlers:
- For newer kernels with aio, let the kernel use its default read
and write operations, and define the aio_read and aio_write operations,
with the AFS specific bits, calling into generic_file_aio_read/write.
The kernels default read/write operations are just wrappers around the
aio versions.
- For older kernels, leave things as is, pointing read and write to
afs_linux_read/write

Change-Id: I2a12c6716dc1974683c2582eb9ada5f841067dd9
Reviewed-on: http://gerrit.openafs.org/4563
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>