completions: uninline try_wait_for_completion and completion_done

m68k fails to build with these functions inlined in completion.h.  Move
them out of line into sched.c and export them to avoid this problem.

Signed-off-by: Dave Chinner <david@fromorbit.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>