posix_acl: xattr representation cleanups

Remove the unnecessary typedefs and the zero-length a_entries array in
struct posix_acl_xattr_header.

Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/fs/btrfs/send.c b/fs/btrfs/send.c
index a87675f..1379e59 100644
--- a/fs/btrfs/send.c
+++ b/fs/btrfs/send.c
@@ -4329,7 +4329,7 @@
 	int ret;
 	struct send_ctx *sctx = ctx;
 	struct fs_path *p;
-	posix_acl_xattr_header dummy_acl;
+	struct posix_acl_xattr_header dummy_acl;
 
 	p = fs_path_alloc();
 	if (!p)