IB/uverbs: Expose UAPI to query MR

Expose UAPI to query MR, this will let user space application that
didn't allocate the MR but has access to by owning the matching command
FD to retrieve its information.

Link: https://lore.kernel.org/r/20200630093916.332097-8-leon@kernel.org


Signed-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>