[linux-cifs-client] directio cifs more than doubled loopback file copy (write) perf

[Steve French]

Trying direct io in loopback network connection (cifs and samba on the
same box).

The time for dd if=/dev/zero of=/mnt/testfile bs=256K count=200

went from more than 1.3 seconds on average without directio to 0.5
seconds with mount -o directio

I did have a minor bug in the code to config it (I had marked an ifdef
CIFS_EXPERIMENTAL instead of CONFIG_CIFS_EXPERIMENTAL) and will fix that
and try to get the performance up a bit higher by eliminating a copy and
sending more than 16K on writes.