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 #13133