Btrfs: should add a permission check for setfacl

On btrfs, do the following
------------------
# su user1
# cd btrfs-part/
# touch aaa
# getfacl aaa
# file: aaa
# owner: user1
# group: user1
user::rw-
group::rw-
other::r--
# su user2
# cd btrfs-part/
# setfacl -m u::rwx aaa
# getfacl aaa
# file: aaa
# owner: user1
# group: user1
user::rwx <- successed to setfacl
group::rw-
other::r--
------------------
but we should prohibit it that user2 changing user1's acl.
In fact, on ext3 and other fs, a message occurs:
setfacl: aaa: Operation not permitted

This patch fixed it.
Signed-off-by: Shi Weihua <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/acl.c

···
 	int ret;
 	struct posix_acl *acl = NULL;
 
+	if (!is_owner_or_cap(dentry->d_inode))
+		return -EPERM;
+
 	if (value) {
 		acl = posix_acl_from_xattr(value, size);
 		if (acl == NULL) {