powerpc/spufs: Use llseek in all file operations

The default for llseek is changing, so we need
explicit operations everywhere.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Jeremy Kerr <jk@ozlabs.org>
Cc: linuxppc-dev@ozlabs.org
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>