ext4: make dioread_nolock the default

This fixes the direct I/O versus writeback race which can reveal stale
data, and it improves the tail latency of commits on slow devices.

Link: https://lore.kernel.org/r/20200125022254.1101588-1-tytso@mit.edu


Signed-off-by: Theodore Ts'o <tytso@mit.edu>