include/linux/kfifo.h: fix INIT_KFIFO()

DECLARE_KFIFO creates a union with a struct kfifo and a buffer array with
size [size + sizeof(struct kfifo)].

INIT_KFIFO then sets the buffer pointer in struct kfifo to point to the
beginning of the buffer array which means that the first call to kfifo_in
will overwrite members of the struct kfifo.

Signed-off-by: David Härdeman <david@hardeman.nu>
Acked-by: Stefani Seibold <stefani@seibold.net>
Cc: <stable@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>