[submodule]: update sonic-linux-kernel

* e9f0369 2018-09-07 | import upstream optoe chunk offset fix (#60) (HEAD, origin/201807) [lguohan]
* b9135e2 2018-08-22 | Fix calculation of page_len in sff_8436_read_write function (#57) [yurypm]

Signed-off-by: Guohan Lu <gulv@microsoft.com>