io_uring: handle INTR error from write

It is possible to get an EINTR error from io_uring when copying cqes.
xev already handles EINTR automatically when it occurs during
io_uring_enter, but doesn't handle it if an individual cqe copy has this
error.

Automatically rearm the completion when the result is EINTR.

src/backend/io_uring.zig

@@ -744,6 +744,8 @@ pub const Completion = struct {
                     @intCast(res)
                 else switch (@as(posix.E, @enumFromInt(-res))) {
                     .CANCELED => error.Canceled,
+                    // If a write is interrupted, we retry it automatically.
+                    .INTR => return .rearm,
                     else => |errno| posix.unexpectedErrno(errno),
                 },
             },