Motivation:
In order NFS server to support extended attributes, they should be exposed
by under laying vfs interface.

Modification:
introduce VirtualFileSystem#get/set/list/removeXattr methods with default
implementations. Update PseudoFs to check permissions and bypass
requests to backing implementation.

To read or list extended attributes, read permission on the file is required
To set or remove extended attributes, write permissions on the file is required

Result:
ground work for extended attribute support.

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