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>
BUG=b:144741353
TEST=xfstests/smoke, no new failures introduced
SOURCE=UPSTREAM(244adf6426ee31a83f397b700d964cff12a247d3)
Change-Id: I0a48200f275e5aaae99c3a249c9fdf202a292097
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/lakitu-kernel/+/2096826
Reviewed-by: Roy Yang <royyang@google.com>
Reviewed-by: Theodore Tso <tytso@google.com>
Tested-by: Harshad Shirwadkar <harshads@google.com>