NFS clients or users of the handle ioctls can pass us arbitrary inode
numbers through the exportfs interface.  Make sure we use the
XFS_IGET_BULKSTAT so that these don't cause shutdowns due to the corruption
checks.  Also translate the EINVAL we get back for invalid inode clusters
into an ESTALE which is more appropinquate, and remove the useless check
for a NULL inode on a successfull xfs_iget return.

I have a testcase to reproduce this using the handle interface which
I will submit to xfsqa.
Reported-by: Mario Becroft <mb@gem.win.co.nz>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>