[XFS] convert xfs to use ERR_CAST

Looks like somehow xfs got missed in the conversion that took place in
e231c2ee, "Convert ERR_PTR(PTR_ERR(p))
instances to ERR_CAST(p)
<http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit


diff;h=e231c2ee>"

SGI-PV: 981498

SGI-Modid: xfs-linux-melb:xfs-kern:31757a

Signed-off-by: Eric Sandeen <sandeen@sandeen.net>
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Niv Sardi <xaiki@sgi.com>
Signed-off-by: Christoph Hellwig <hch@infradead.org>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>