IB/mthca: Free correct MPT on error exit from mthca_fmr_alloc()

When mthca_fmr_alloc() returns an error, it should free the MPT at the
index key, not mr->ibmr.lkey, since the lkey has been mangled by
hw_index_to_key() and no longer is the real index.  This bug causes
corruption of the MPT table free bitmap when mthca_fmr_alloc() fails.

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