Added error for writing to /dev/ on Linux

Starting in Linux 5.10, trying to write to /dev/{null,zero} errors out.
Prefer to inform people when this happens rather than hoping they guess
what's wrong.

Reviewed-by: Antonio Russo <aerusso@aerusso.net>
Reviewed-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes:  #11991

lib/libzfs/libzfs_sendrecv.c

@@ -847,7 +847,8 @@ dump_ioctl(zfs_handle_t *zhp, const char *fromsnap, uint64_t fromsnap_obj,
 		case ERANGE:
 		case EFAULT:
 		case EROFS:
-			zfs_error_aux(hdl, strerror(errno));
+		case EINVAL:
+			zfs_error_aux(hdl, "%s", strerror(errno));
 			return (zfs_error(hdl, EZFS_BADBACKUP, errbuf));
 
 		default: