IB/mthca: Return proper error codes from mthca_fmr_alloc()

If the allocation of the MTT or the mailbox failed, mthca_fmr_alloc()
would return 0 (success) no matter what. This leads to crashes a
little down the road, when we try to dereference eg mr->mtt, which was
really ERR_PTR(-Ewhatever).

Signed-off-by: Olaf Kirch <olaf.kirch@oracle.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>