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

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>