Linux: Don't roll our own llseek

Linux: Don't roll our own llseek

llseek is a little more complex than just setting the pos field of
the file structure.

Where vfs_llseek is available to us (it has been since 2.6.8) use that
directly. If vfs_llseek isn't available, then use either the
filesystem's llseek method, or default_llseek (available since 2.6.0),
to ensure we get the proper locking.

Change-Id: I26bcbff984c7164be1724eee7a46cbe6e02bc510
Reviewed-on: http://gerrit.openafs.org/1816
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>