ext4: Mark the buffer_heads as dirty and uptodate after prepare_write

(cherry picked from commit ed9b3e3379731e9f9d2f73f3d7fd9e7d2ce3df4a)

We need to make sure we mark the buffer_heads as dirty and uptodate
so that block_write_full_page write them correctly.

This fixes mmap corruptions that can occur in low memory situations.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>