Don't count embedded bps in read stats

Currently, ZFS tracks statistics about calls to arc_read()
via the /proc/spl/kstat/zfs/<pool>/reads file for debugging.
Unfortunately, this file currently counts embedded bps as
disk reads since they are technically processed by the ZIO
layer. This pollutes the log since the ARC will never cache
embedded bps. This patch corrects this issue by preventing
the logging of embedded bp reads.

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #7334