Illumos #175: zfs vdev cache consumes excessive memory

Illumos #175: zfs vdev cache consumes excessive memory

Note that with the current ZFS code, it turns out that the vdev
cache is not helpful, and in some cases actually harmful.  It
is better if we disable this.  Once some time has passed, we
should actually remove this to simplify the code.  For now we
just disable it by setting the zfs_vdev_cache_size to zero.
Note that Solaris 11 has made these same changes.

References to Illumos issue and patch:
- https://www.illumos.org/issues/175
- https://github.com/illumos/illumos-gate/commit/b68a40a845

Reviewed by: George Wilson <george.wilson@delphix.com>
Reviewed by: Eric Schrock <eric.schrock@delphix.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #340