mm/kmemleak.c: make kmemleak_boot_config() __init

The early_param() is only called during kernel initialization, So Linux
marks the functions of it with __init macro to save memory.

But it forgot to mark the kmemleak_boot_config().  So, Make it __init as
well.

Link: http://lkml.kernel.org/r/20180117034720.26897-1-douly.fnst@cn.fujitsu.com
Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>