[linux-cifs-client] direct writes bypassing the cache

[Steve French]

I added a "direct" mount option to the cifs client (only enabled if the
CIFS_EXPERIMENTAL config option is chosen)

With this enabled the CIFS client would send 16K writes (instead of 4K)
when the application requested (reads could already go that large but
they too could max out at 16K).  I will look at using large_readx and
large_writex so we can go bigger still.  There are a few cases, large
sequential series of reads or writes where this mount option might make
sense.

This change was less complicated than honoring the O_DIRECT flag on open
as NFS did, but I eventually plan to get around to that.   Large reads
and writes need to be an option for
decent performance on gige when there are few processes on few clients
talking to the server (and where the long latency compared to transfer
speed would otherwise cause much
bandwidth waste).