Fix embedded bp accounting in count_block()

Currently, count_block() does not correctly account for the
possibility that the bp that is passed to it could be embedded.
These blocks shouldn't be counted since the work of scanning
these blocks in already handled when the containing block is
scanned. This patch simply resolves this issue by returning
early in this case.

Reviewed by: Allan Jude <allanjude@freebsd.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Authored-by: Bill Sommerfeld <sommerfeld@alum.mit.edu>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8800
Closes #8766