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: NIvan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Ollie Wild <aaw@google.com>
Acked-by: NPeter 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: NLinus Torvalds <torvalds@linux-foundation.org>

fs/binfmt_aout.c

@@ -319,7 +319,6 @@ static int load_aout_binary(struct linux_binprm * bprm, struct pt_regs * regs)
 	current->mm->free_area_cache = current->mm->mmap_base;
 	current->mm->cached_hole_size = 0;
 
-	current->mm->mmap = NULL;
 	compute_creds(bprm);
  	current->flags &= ~PF_FORKNOEXEC;
 #ifdef __sparc__