RDMA/siw: Use the common mmap_xa helpers

RDMA/siw: Use the common mmap_xa helpers

Remove the functions related to managing the mmap_xa database.  This code
is now common in ib_core.

Link: https://lore.kernel.org/r/20191030094417.16866-6-michal.kalderon@marvell.com
Signed-off-by: Ariel Elior <ariel.elior@marvell.com>
Signed-off-by: Michal Kalderon <michal.kalderon@marvell.com>
Signed-off-by: Bernard Metzler <bmt@zurich.ibm.com>
Reviewed-by: Bernard Metzler <bmt@zurich.ibm.com>
Tested-by: Bernard Metzler <bmt@zurich.ibm.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>