kernel/kallsyms.c: use __seq_open_private()

Reduce boilerplate code by using __seq_open_private() instead of
seq_open() in kallsyms_open().

Signed-off-by: Rob Jones <rob.jones@codethink.co.uk>
Cc: Gideon Israel Dsouza <gidisrael@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>