[XFS] fix instant oops with tracing enabled

We can only read inode->i_count if the inode is actually there and not a
NULL pointer. This was introduced in one of the recent sync patches.

SGI-PV: 988255

SGI-Modid: xfs-linux-melb:xfs-kern:32315a

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