RDMA/core: Remove unused header files mm.h, socket.h, scatterlist.h

Structures of ib_verbs.h don't use fields/structures of mm.h, socket.h or
scatterlist.h.  So remove such header files inclusion.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>