RDMA/hns: Fix mismatch exception rollback

When dma_alloc_coherent() fails in hns_roce_alloc_hem(), just call
kfree() to release hem instead of hns_roce_free_hem().

Fixes: c00743cb

 ("RDMA/hns: Simplify 'struct hns_roce_hem' allocation")
Signed-off-by: wenglianfa <wenglianfa@huawei.com>
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>
Link: https://lore.kernel.org/r/20240412091616.370789-7-huangjunxian6@hisilicon.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>