ext4: don't set dioread_nolock by default for blocksize < pagesize

Currently on calling echo 3 > drop_caches on host machine, we see
FS corruption in the guest. This happens on Power machine where
blocksize < pagesize.

So as a temporary workaound don't enable dioread_nolock by default
for blocksize < pagesize until we identify the root cause.

Also emit a warning msg in case if this mount option is manually
enabled for blocksize < pagesize.

Reported-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Link: https://lore.kernel.org/r/20200327200744.12473-1-riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>