Rename fallthrough to zfs_fallthrough

Unfortunately macOS has obj-C keyword "fallthrough" in the OS headers.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Damian Szuberski <szuberskidamian@gmail.com>
Signed-off-by: Jorgen Lundman <lundman@lundman.net>
Closes #13097

lib/libzfs/libzfs_dataset.c

@@ -1363,7 +1363,7 @@ badlabel:
 				(void) zfs_error(hdl, EZFS_BADPROP, errbuf);
 				goto error;
 			}
-			fallthrough;
+			zfs_fallthrough;
 		}
 
 		case ZFS_PROP_SHARESMB:
@@ -3783,7 +3783,7 @@ 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;
+			zfs_fallthrough;
 #endif
 		default:
 			return (zfs_standard_error(hdl, errno, errbuf));

lib/libzfs/libzfs_pool.c

@@ -316,7 +316,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
 				    len);
 				break;
 			}
-			fallthrough;
+			zfs_fallthrough;
 		default:
 			(void) strlcpy(buf, "-", len);
 			break;
@@ -407,7 +407,7 @@ zpool_get_prop(zpool_handle_t *zhp, zpool_prop_t prop, char *buf,
 				(void) snprintf(buf, len, "-");
 				break;
 			}
-			fallthrough;
+			zfs_fallthrough;
 		default:
 			(void) snprintf(buf, len, "%llu", (u_longlong_t)intval);
 		}

lib/libzfs/libzfs_sendrecv.c

@@ -4947,7 +4947,7 @@ zfs_receive_one(libzfs_handle_t *hdl, int infd, const char *tosnap,
 				(void) zfs_error(hdl, EZFS_BUSY, errbuf);
 				break;
 			}
-			fallthrough;
+			zfs_fallthrough;
 		default:
 			(void) zfs_standard_error(hdl, ioctl_errno, errbuf);
 		}

lib/libzfs/libzfs_util.c

@@ -600,8 +600,8 @@ zfs_setprop_error(libzfs_handle_t *hdl, zfs_prop_t prop, int err,
 			(void) zfs_error(hdl, EZFS_VOLTOOBIG, errbuf);
 			break;
 		}
+		zfs_fallthrough;
 #endif
-		fallthrough;
 	default:
 		(void) zfs_standard_error(hdl, err, errbuf);
 	}