kref: Inline all functions

These are tiny functions, there's no point in having them out-of-line.

Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/n/tip-8eccvi2ur2fzgi00xdjlbf5z@git.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>