Linux: define llseek operations

Linux: define llseek operations

With kernel 2.6.37 it is now mandatory to define the llseek operation
for files and directories.  If these are not defined, no_llseek is
called, and any attempt to seek returns ESPIPE.

Most file systems use generic_file_llseek, but it seems safer to use
default_llseek which is what the vfs used to call for us by default.
In 2.6.37 these two functions are actually functionally identical.

Change-Id: I7ad736e3953c4feaa51afb9f7ff2760c3140373f
Reviewed-on: http://gerrit.openafs.org/3292
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: BuildBot <buildbot@rampaginggeek.com>