Linux: Don't pass f_pos down to the filesystem

Linux: Don't pass f_pos down to the filesystem

In 2.6.8, Linux shifted from passing f_pos directly to the read
and write routines, and started passing a copy. This helps reduce,
but does not remove, the race issues with f_pos itself. Make this
change for us.

Take the opportunity to remove the uneccessary macros, and tidy up
some casting.

Change-Id: I3b4cdf1e6e8127cbe0055829605268953c4397a6
Reviewed-on: http://gerrit.openafs.org/1818
Reviewed-by: Marc Dionne <marc.c.dionne@gmail.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>