[XFS] Check for invalid flags in xfs_attrlist_by_handle.

xfs_attrlist_by_handle should only take the ATTR_ flags for the root
namespaces. The ATTR_KERN* flags may change at anytime and expect special
preconditions that can't be guaranteed for userspace-originating requests.
For example passing down ATTR_KERNNOVAL through xfs_attrlist_by_handle
will hit an assert in debug builds currently.

SGI-PV: 983677

SGI-Modid: xfs-linux-melb:xfs-kern:31351a

Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: Tim Shimmin <tes@sgi.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>