Rationalise some #ifdefs in the LINUX osi layer

Rationalise some #ifdefs in the LINUX osi layer

Introduce osi_compat.h, a place to hide compatibility #ifdefs. Make
a start on using this by purging #ifdefs from osi_vnodeops, and pushing
changes into inline compatibility functions.

The general idea here is to create a single location where we can do our
multiple version compatibility magic, and by doing so reduce the
complexity of the rest of the code. The approach is that where the
kernel introduces a new interface, we should switch to using it in the
rest of the code, and add a compatibility function (in osi_compat.h)
which implements the new interface in terms of the old one, suitably
protected by #ifdefs.

Where an interface changes signature, but not name, we should implement
an afs_<blah> version of the function which has the signature of the
newest version, and within that call out to the real kernel versions,
mapping between the different parameters as appropriate.

Reviewed-on: http://gerrit.openafs.org/568
Tested-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Derrick Brashear <shadow@dementia.org>