can_sleep is always 0 when iboib_mcast_alloc() is called, so remove it and
use GFP_ATOMIC instead of GFP_KERNEL.

Link: https://lore.kernel.org/r/20200525130305.171509-1-kamalheib1@gmail.comSigned-off-by: Kamal Heib <kamalheib1@gmail.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>