Revert "Temporarily disable Direct IO by default"

This partially reverts commit 41210597. Now that b4e4cbeb2 has
been merged Direct IO can be enabled by default for Linux, but
for FreeBSD there still remains a potentially insufficient range
locking in zfs_getpages() which needs to be resolved.

Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #16629