IB/uverbs: Expose device memory capabilities to user

Adding a new capability field under ib_uverbs_ex_query_device_resp -
max_dm_size - which reflects the maximum amount of device memory
that is available for allocation on a device in bytes.

Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>