ovl: listxattr: use strnlen()

[ Upstream commit 7cb35119d067191ce9ebc380a599db0b03cbd9d9 ]

Be defensive about what underlying fs provides us in the returned xattr
list buffer.  If it's not properly null terminated, bail out with a warning
insead of BUG.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>