Use fallthrough macro

As of the Linux 5.9 kernel a fallthrough macro has been added which
should be used to anotate all intentional fallthrough paths.  Once
all of the kernel code paths have been updated to use fallthrough
the -Wimplicit-fallthrough option will because the default.  To
avoid warnings in the OpenZFS code base when this happens apply
the fallthrough macro.

Additional reading: https://lwn.net/Articles/794944/

Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #12441

lib/libzfs/libzfs_dataset.c

@@ -1363,10 +1363,9 @@ badlabel:
 				(void) zfs_error(hdl, EZFS_BADPROP, errbuf);
 				goto error;
 			}
+			fallthrough;
 		}
 
-			/*FALLTHRU*/
-
 		case ZFS_PROP_SHARESMB:
 		case ZFS_PROP_SHARENFS:
 			/*
@@ -3765,8 +3764,8 @@ zfs_create(libzfs_handle_t *hdl, const char *path, zfs_type_t type,
 			if (type == ZFS_TYPE_VOLUME)
 				return (zfs_error(hdl, EZFS_VOLTOOBIG,
 				    errbuf));
+			fallthrough;
 #endif
-			/* FALLTHROUGH */
 		default:
 			return (zfs_standard_error(hdl, errno, errbuf));
 		}

lib/libzfs/libzfs_pool.c

@@ -314,7 +314,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
 				    len);
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			(void) strlcpy(buf, "-", len);
 			break;
@@ -405,7 +405,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
 				(void) snprintf(buf, len, "-");
 				break;
 			}
-			/* FALLTHROUGH */
+			fallthrough;
 		default:
 			(void) snprintf(buf, len, "%llu", (u_longlong_t)intval);
 		}

lib/libzfs/libzfs_sendrecv.c

@@ -4878,7 +4878,7 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 				(void) zfs_error(hdl, EZFS_BUSY, errbuf);
 				break;
 			}
-			/* fallthru */
+			fallthrough;
 		default:
 			(void) zfs_standard_error(hdl, ioctl_errno, errbuf);
 		}

lib/libzfs/libzfs_util.c

@@ -598,7 +598,7 @@ zfs_setprop_error(libzfs_handle_t *hdl, zfs_prop_t prop, int err,
 			break;
 		}
 #endif
-		/* FALLTHROUGH */
+		fallthrough;
 	default:
 		(void) zfs_standard_error(hdl, err, errbuf);
 	}