ecryptfs: Make inode bdi consistent with superblock bdi

commit 985ca0e6 upstream.

Make the inode mapping bdi consistent with the superblock bdi so that
dirty pages are flushed properly.

Signed-off-by: Thieu Le <thieule@chromium.org>
Signed-off-by: Tyler Hicks <tyhicks@linux.vnet.ibm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>