IB/mthca: Simplify use of size0 in work request posting

    
Current code sets size0 to 0 at the start of work request posting
functions and then handles size0 == 0 specially within the loop over
work requests.  Change this so size0 is set along with f0 the first
time through the loop (when nreq == 0).  This makes the code easier to
understand by making it clearer that f0 and size0 are always
initialized if nreq != 0 without having to know that size0 == 0
implies nreq == 0.

Also annotate size0 with uninitialized_var() so that this doesn't
introduce a new compiler warning.

Signed-off-by: Roland Dreier <rolandd@cisco.com>