It's currently possible to bypass xattr namespace access rules by
prefixing valid xattr names with "os2.", since the os2 namespace stores
extended attributes in a legacy format with no prefix.

This patch adds checking to deny access to any valid namespace prefix
following "os2.".
Signed-off-by: NDave Kleikamp <shaggy@linux.vnet.ibm.com>
Reported-by: NSergey Vlasov <vsu@altlinux.ru>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>