IB/core: Remove redundant pointer mm

The pointer mm is assigned a value but it is never used. The pointer is
redundant and can be removed.

Link: https://lore.kernel.org/r/20211204234904.105026-1-colin.i.king@gmail.com


Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>