dm era: Use correct value size in equality function of writeset tree

commit 64f2d15afe7b336aafebdcd14cc835ecf856df4b upstream.

Fix the writeset tree equality test function to use the right value size
when comparing two btree values.

Fixes: eec40579 ("dm: add era target")
Cc: stable@vger.kernel.org # v3.15+
Signed-off-by: Nikos Tsironis <ntsironis@arrikto.com>
Reviewed-by: Ming-Hung Tsai <mtsai@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>