LU-15593 mdt: Add option to disable use of SOM

LU-15593 mdt: Add option to disable use of SOM

There's currently no way to disable use of strict SOM,
which is a problem if there's ever a SOM bug.  This is
tricky to do from the client, but easy on the MDT.

The test just verifies that the size stays the same with
strict SOM disabled, because there's no easy way to check
SOM is disabled unless SOM is broken.  (Since it gives the
same value for stat.)

Note this patch requires LU-15609 to be fixed on the client
or the client will see size as 0 when SOM is disabled.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I338fa7e4dd423b07df0f3c5bad4ec6f02e935fea