ext4: verify unwritten extent conversion in buff-io

ext4: verify unwritten extent conversion in buff-io

There was an issue where with filesize > 4G, map.m_lblk
was getting overflow in buff-IO path while converting unwritten to
written extent with dioread_nolock mount option with bs < ps.

Adding a testcase to test for such regressions with dioread_nolock
mount option. To reproduce the same regression w/o the fix
in the kernel, test with bs < ps config.

The following commit fixed the issue in linux.

id1e18b8824dd ("ext4: fix bs < ps issue reported with dioread_nolock
mount opt")

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>