signalfd.2: Use sizeof consistently

Use ``sizeof`` consistently through all the examples in the following
way:

- Use the name of the variable instead of its type as argument for
  ``sizeof``.

	Rationale:
	https://www.kernel.org/doc/html/v5.8/process/coding-style.html#allocating-memory

Signed-off-by: Alejandro Colomar <colomar.6.4.3@gmail.com>
Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>

man2/signalfd.2

@@ -502,8 +502,8 @@ main(int argc, char *argv[])
         handle_error("signalfd");
 
     for (;;) {
-        s = read(sfd, &fdsi, sizeof(struct signalfd_siginfo));
-        if (s != sizeof(struct signalfd_siginfo))
+        s = read(sfd, &fdsi, sizeof(fdsi));
+        if (s != sizeof(fdsi))
             handle_error("read");
 
         if (fdsi.ssi_signo == SIGINT) {