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: NAneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>