RDMA/addr: Mark addr_resolve as might_sleep()

Under one path through ib_nl_fetch_ha() this calls nlmsg_new(GFP_KERNEL)
which is a sleeping call. This is a very rare path, so mark fetch_ha() and
the module external entry point that conditionally calls through to
fetch_ha() as might_sleep().

Link: https://lore.kernel.org/r/20200506074701.9775-2-leon@kernel.org
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>