Add ZFS perf test for dbuf cache

Add ZFS perf test for dbuf cache

This change adds a test for sequential reads out of the dbuf cache.
It's essentially a copy of sequential_reads_cached, using a smaller
data set. The sequential read tests are renamed to differentiate them.

Authored by: Dan Kimmel <dan.kimmel@delphix.com>
Reviewed by: Paul Dagnelie <pcd@delphix.com>
Reviewed by: Matt Ahrens <mahrens@delphix.com>
Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: John Wren Kennedy <john.kennedy@delphix.com>
Closes #7225