lkdtm: use generic_file_llseek in debugfs

When the default llseek behavior gets changed to
not allowing seek, all file operations that rely
on the current behaviour need to use an explicit
.llseek operation.

The files that lkdtm uses in debugfs are regular
files and they get read using simple_read_from_buffer,
so generic_file_llseek is the right operation.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>