ashmem: Add shmem_set_file to mm/shmem.c

NOT FOR STAGING
This patch re-adds the original shmem_set_file to mm/shmem.c
and converts ashmem.c back to using it.

CC: Brian Swetland <swetland@google.com>
CC: Colin Cross <ccross@android.com>
CC: Arve Hjønnevåg <arve@android.com>
CC: Dima Zavin <dima@android.com>
CC: Robert Love <rlove@google.com>
CC: Greg KH <greg@kroah.com>
Signed-off-by: John Stultz <john.stultz@linaro.org>
diff --git a/include/linux/mm.h b/include/linux/mm.h
index d8738a4..fa9f800 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -873,6 +873,7 @@
 
 int shmem_lock(struct file *file, int lock, struct user_struct *user);
 struct file *shmem_file_setup(const char *name, loff_t size, unsigned long flags);
+void shmem_set_file(struct vm_area_struct *vma, struct file *file);
 int shmem_zero_setup(struct vm_area_struct *);
 
 extern int can_do_mlock(void);