smb: client: let destroy_mr_list() call ib_dereg_mr() before ib_dma_unmap_sg()

This is more consistent as we call ib_dma_unmap_sg() only
when the memory is no longer registered.

This is the same pattern as calling ib_dma_unmap_sg() after
IB_WR_LOCAL_INV.

Fixes: c7398583 ("CIFS: SMBD: Implement RDMA memory registration")
Cc: Steve French <smfrench@gmail.com>
Cc: Tom Talpey <tom@talpey.com>
Cc: Long Li <longli@microsoft.com>
Cc: Namjae Jeon <linkinjeon@kernel.org>
Cc: linux-cifs@vger.kernel.org
Cc: samba-technical@lists.samba.org
Signed-off-by: Stefan Metzmacher <metze@samba.org>
Signed-off-by: Steve French <stfrench@microsoft.com>