Cap maximum aggregate IO size

Cap maximum aggregate IO size

Commit 8542ef8 allowed optional IOs to be aggregated beyond
the specified aggregation limit.  Since the aggregation limit
was also used to enforce the maximum block size, setting
`zfs_vdev_aggregation_limit=16777216` could result in an
attempt to allocate an ABD larger than 16M.

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6259
Closes #6270