IB/qib: Use generic_file_llseek

When the default llseek action gets changed to no_llseek, all file
systems relying on the current behaviour need to set explicit .llseek
operations.

In case of qib_fs, we want the files to be seekable, so
generic_file_llseek fits best.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Roland Dreier <rolandd@cisco.com>