Motivation:
As client might chack for support of extended attribute, let server
nicely reply 'not supported'

Modification:
introduce FATTR4_XATTR_SUPPORT bit in bitmask of support attributes.
Reply hard-coded 'not supported' as long as backend file system can't be
queried for this information.

Result:
client can auto-discover (not yet) that whatever extended attributes are
supported or not.

Acked-by: Lea Morschel
Acked-by: Paul Millar
Target: master