Fix positional I/O support for largefiles

Fix positional I/O support for largefiles

Commit 335ccb40 introduced positional I/O support for the fileserver,
but didn't handle the largefile versions of preadv and pwritev. As a
result, the fileserver cannot handle files of more than 2Gb in size.

Fix this by using preadv64 and pwritev64 where O_LARGEFILE is defined,
in the same way as for all other I/O primitives.

Reviewed-by: Simon Wilkinson <sxw@inf.ed.ac.uk>
Reviewed-on: http://gerrit.openafs.org/4152
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>
(cherry picked from commit bbf406da30428a22b4a6d28a7d36dbb786013cfa)

Change-Id: I5d15097e4c9aa83444520628f484407d244ddf97
Reviewed-on: http://gerrit.openafs.org/4161
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementia.org>