ocfs2: remove duplicate definition in xattr

Include/linux/xattr.h already has the definition about xattr prefix,
so remove the duplicate definitions in xattr.c.

Signed-off-by: Tiger Yang <tiger.yang@oracle.com>
Signed-off-by: Joel Becker <joel.becker@oracle.com>
Signed-off-by: Mark Fasheh <mfasheh@suse.com>