smb: client: call ib_dma_unmap_sg if mr->sgt.nents is not 0

This seems to be the more reliable way to check if we need to
call ib_dma_unmap_sg().

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>