[PATCH] IB/mthca: fix MR allocation error path

Fix error handling in MR allocation for mem-free mode: mthca_free must get an
MR index, not a key.

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <roland@topspin.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>