Introduce create/destroy SRQ commands over the ioctl interface to let it
be extended to get an asynchronous event FD.

Link: https://lore.kernel.org/r/20200519072711.257271-6-leon@kernel.orgSigned-off-by: Yishai Hadas <yishaih@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>