[XFS] Unlock inode before calling xfs_idestroy()

Lock debugging reported the ilock was being destroyed without being
unlocked. We don't need to lock the inode until we are going to insert it
into the radix tree.

SGI-PV: 987246

SGI-Modid: xfs-linux-melb:xfs-kern:32159a

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