RDMA/erdma: Using the key in FMR WR instead of MR structure

RDMA driver should get the MR key from FMR WR, not the MR structure passed
in.

Fixes: 15505577

 ("RDMA/erdma: Add verbs implementation")
Link: https://lore.kernel.org/r/20220810014320.88026-2-chengyou@linux.alibaba.com
Signed-off-by: Cheng Xu <chengyou@linux.alibaba.com>
Signed-off-by: Leon Romanovsky <leon@kernel.org>