[XFS] Remove unused variable in ktrace_free()

entries_size is probably left over from when we used to pass the
size to kmem_free().

Signed-off-by: Lachlan McIlroy <lachlan@sgi.com>
Reviewed-by: Christoph Hellwig <hch@infradead.org>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>