[XFS] cleanup vnode useage in xfs_ioctl.c

xfs_ioctl.c passes around vnode pointers quite a lot, but all places
already have the Linux inode which is identical to the vnode these days.
Clean the code up to always use the Linux inode.

SGI-PV: 971186
SGI-Modid: xfs-linux-melb:xfs-kern:29807a

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