ext4: fix reserved cluster accounting at delayed write time

The code in ext4_da_map_blocks sometimes reserves space for more
delayed allocated clusters than it should, resulting in premature
ENOSPC, exceeded quota, and inaccurate free space reporting.

Fix this by checking for written and unwritten blocks shared in the
same cluster with the newly delayed allocated block.  A cluster
reservation should not be made for a cluster for which physical space
has already been allocated.

Signed-off-by: Eric Whitney <enwlinux@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>