Revert "Temporarily disable Direct IO by default"

This partially reverts commit 4121059.  Now that b4e4cbe 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.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

module/zfs/zfs_vnops.c

@@ -77,8 +77,15 @@ static int zfs_bclone_wait_dirty = 0;
  * Enable Direct I/O. If this setting is 0, then all I/O requests will be
  * directed through the ARC acting as though the dataset property direct was
  * set to disabled.
+ *
+ * Disabled by default on FreeBSD until a potential range locking issue in
+ * zfs_getpages() can be resolved.
  */
+#ifdef __FreeBSD__
 static int zfs_dio_enabled = 0;
+#else
+static int zfs_dio_enabled = 1;
+#endif
 
 
 /*