Fix occasional rsend test crashes

We have occasional crashes in the rsend tests. Debugging revealed 
that this is because the send_worker thread is getting EINTR from 
splice(). This happens when a non-fatal signal is received during 
the syscall. We should retry the syscall, rather than exiting failure.
Tweak the loop to only break if the splice is finished or we receive 
a non-EINTR error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: Paul Dagnelie <pcd@delphix.com>
Closes #15273

lib/libzfs_core/libzfs_core.c

@@ -650,10 +650,12 @@ send_worker(void *arg)
 	unsigned int bufsiz = max_pipe_buffer(ctx->from);
 	ssize_t rd;
 
-	while ((rd = splice(ctx->from, NULL, ctx->to, NULL, bufsiz,
-	    SPLICE_F_MOVE | SPLICE_F_MORE)) > 0)
-		;
-
+	for (;;) {
+		rd = splice(ctx->from, NULL, ctx->to, NULL, bufsiz,
+		    SPLICE_F_MOVE | SPLICE_F_MORE);
+		if ((rd == -1 && errno != EINTR) || rd == 0)
+			break;
+	}
 	int err = (rd == -1) ? errno : 0;
 	close(ctx->from);
 	return ((void *)(uintptr_t)err);