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/libzfsbootenv/lzbe_device.c

@@ -63,7 +63,7 @@ lzbe_set_boot_device(const char *pool, lzbe_flags_t flag, const char *device)
 			/* Drop this nvlist */
 			fnvlist_free(nv);
 		}
-		fallthrough;
+		zfs_fallthrough;
 	case lzbe_replace:
 		nv = fnvlist_alloc();
 		break;