mm: fix exit_mmap BUG() on a.out binary exit

The problem was introduced by commit "mm: variable length argument
support" (b6a2fea3

)
as it didn't update fs/binfmt_aout.c like other binfmt's.

I noticed that on alpha when accidentally launched old OSF/1
Acrobat Reader binary. Obviously, other architectures are affected
as well.

Signed-off-by: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Ollie Wild <aaw@google.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Hugh Dickins <hugh@veritas.com>
Cc: Adrian Bunk <bunk@stusta.de>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>