libzfs_core: lzc_send_wraper: maximise pipe buffer for existing pipes

This reduces scheduler overhead by letting the reader consume bigger
chunks (64k => 128k at full throttle)

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Paul Dagnelie <pcd@delphix.com>
Reviewed-by: Rich Ercolani <rincebrain@gmail.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes openzfs#13133

lib/libzfs_core/libzfs_core.c

@@ -652,8 +652,9 @@ send_worker(void *arg)
  * Returns the error from func(), if nonzero,
  * otherwise the error from the thread.
  *
- * No-op if orig_fd is -1, already a pipe, and on not-Linux;
- * as such, it is safe to wrap/call wrapped functions in a wrapped context.
+ * No-op if orig_fd is -1, already a pipe (but the buffer size is bumped),
+ * and on not-Linux; as such, it is safe to wrap/call wrapped functions
+ * in a wrapped context.
  */
 int
 lzc_send_wrapper(int (*func)(int, void *), int orig_fd, void *data)
@@ -662,8 +663,11 @@ lzc_send_wrapper(int (*func)(int, void *), int orig_fd, void *data)
 	struct stat sb;
 	if (orig_fd != -1 && fstat(orig_fd, &sb) == -1)
 		return (errno);
-	if (orig_fd == -1 || S_ISFIFO(sb.st_mode))
+	if (orig_fd == -1 || S_ISFIFO(sb.st_mode)) {
+		if (orig_fd != -1)
+			(void) max_pipe_buffer(orig_fd);
 		return (func(orig_fd, data));
+	}
 	if ((fcntl(orig_fd, F_GETFL) & O_ACCMODE) == O_RDONLY)
 		return (errno = EBADF);