Clear QUEUE_FLAG_ADD_RANDOM on zvols

zvols should not be an entropy source for the kernel. Disable it to be
consistent with the upstream kernel.

torvalds/linux@b277da0a8a594308e17881f4926879bd5fca2a2d

Signed-off-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3713