__exit_mm() is an inlined version of exit_mm().  This patch unifies them.

Saves 356 byte in exit.o.
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>