Linux: Handle llseek failure

Linux: Handle llseek failure

If llseek fails, then we return immediately, potentially leaving the
wrong address space configured for this process, and without resetting
the processes saved limits.

Fix this by creating a common exit point for osi_rdwr, and using it.

Change-Id: Ifff8cb0155f92a4130d21828d5ee2d2a55c09195
Reviewed-on: http://gerrit.openafs.org/1155
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Derrick Brashear <shadow@dementia.org>