cifs: smbd: Retry on memory registration failure

Memory registration failure doesn't mean this I/O has failed, it means the
transport is hitting I/O error or needs reconnect. This error is not from
the server.

Indicate this error to upper layer, and let upper layer decide how to
reconnect and proceed with this I/O.
Signed-off-by: Long Li <longli@microsoft.com>
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -3223,7 +3223,7 @@ smb2_new_read_req(void **buf, unsigned int *total_len,
 				rdata->nr_pages, rdata->page_offset,
 				rdata->tailsz, true, need_invalidate);
 		if (!rdata->mr)
-			return -ENOBUFS;
+			return -EAGAIN;
 
 		req->Channel = SMB2_CHANNEL_RDMA_V1_INVALIDATE;
 		if (need_invalidate)
@@ -3628,7 +3628,7 @@ smb2_async_writev(struct cifs_writedata *wdata,
 				wdata->nr_pages, wdata->page_offset,
 				wdata->tailsz, false, need_invalidate);
 		if (!wdata->mr) {
-			rc = -ENOBUFS;
+			rc = -EAGAIN;
 			goto async_writev_out;
 		}
 		req->Length = 0;