Security: Add Hook to test if the particular xattr is part of a MAC model.

Security: Add Hook to test if the particular xattr is part of a MAC model.

The interface to request security labels from user space is the xattr
interface. When requesting the security label from an NFS server it is
important to make sure the requested xattr actually is a MAC label. This allows
us to make sure that we get the desired semantics from the attribute instead of
something else such as capabilities or a time based LSM.

Acked-by: Eric Paris <[email protected]>
Acked-by: James Morris <[email protected]>
Signed-off-by: Matthew N. Dodd <[email protected]>
Signed-off-by: Miguel Rodel Felipe <[email protected]>
Signed-off-by: Phua Eu Gene <[email protected]>
Signed-off-by: Khin Mi Mi Aung <[email protected]>
Signed-off-by: Trond Myklebust <[email protected]>