LINUX24: remove pagecopy and other 2.6-only code

LINUX24: remove pagecopy and other 2.6-only code

The client background readahead code and 'readpages' implementation
were only written for 2.6 kernels and used 2.6-specific constructs.
Remove them, along with references to the 'pagecopy' code, and some
other 2.6-only cruft in osi_vnodeops.c.

Change-Id: I2e939e98bd72cd7b275b0f7a389607ee5dc94efa
Reviewed-on: http://gerrit.openafs.org/2271
Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>