[PATCH] IB/mthca: clean up mthca_dereg_mr()

It's cleaner to kfree mthca_mr, and not rely on the fact that ib_mr is the
first field in mthca_mr.

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>