mm: use mm->exe_file instead of first VM_EXECUTABLE vma->vm_file

Some security modules and oprofile still uses VM_EXECUTABLE for retrieving
a task's executable file.  After this patch they will use mm->exe_file
directly.  mm->exe_file is protected with mm->mmap_sem, so locking stays
the same.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
Acked-by: Chris Metcalf <cmetcalf@tilera.com>			[arch/tile]
Acked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>	[tomoyo]
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Carsten Otte <cotte@de.ibm.com>
Cc: Cyrill Gorcunov <gorcunov@openvz.org>
Cc: Eric Paris <eparis@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Acked-by: James Morris <james.l.morris@oracle.com>
Cc: Jason Baron <jbaron@redhat.com>
Cc: Kentaro Takeda <takedakn@nttdata.co.jp>
Cc: Matt Helsley <matthltc@us.ibm.com>
Cc: Nick Piggin <npiggin@kernel.dk>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Robert Richter <robert.richter@amd.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Venkatesh Pallipadi <venki@google.com>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>