Any node that returns ENOSYS for Getxattr will cause the kernel to
stop issuing Getxattr calls altogether. In file systems that must
support GetXAttr and that has multiple node types, tracking down all
node types and overriding GetXAttr for them individually is annoying.

For filesystems that need the performance benefit of skipping GETXATTR
calls, provide a DisableXAttrs option.

Change-Id: I98327aa959b3b26192e15bc3be96b7a6c0ada5f6