kfifo: fix KFIFO_INIT in include/linux/kfifo.h

include/linux/kfifo.h first defines and then undefines __kfifo_initializer
which is used by INIT_KFIFO (which is also a macro, so building a module
which uses INIT_KFIFO will fail).

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>