[XFS] avoid xfs_getattr in XFS_IOC_FSGETXATTR ioctl

No need to call into xfs_getattr and put a big bhv_vattr_t on the stack
just to get a little information from the XFS inode.

Add a helper called xfs_ioc_fsgetxattr instead that deals with retrieving
the information in a clean way.

SGI-PV: 968563
SGI-Modid: xfs-linux-melb:xfs-kern:29780a

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