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

The problem was introduced by commit "mm: variable length argument
support" (b6a2fea39318e43fee84fa7b0b90d68bed92d2ba)
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 <[e-mail]>
Cc: Ollie Wild <[e-mail]>
Acked-by: Peter Zijlstra <[e-mail]>
Cc: Andrew Morton <[e-mail]>
Cc: Hugh Dickins <[e-mail]>
Cc: Adrian Bunk <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

fs/binfmt_aout.c

···
 	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__