Support secure discard on zvols

Linux 2.6.36 introduced REQ_SECURE to indicate when discards *must* be
processed, such that we cannot do optimizations like block alignment.
Consequently, the discard semantics prior to 2.6.36 require us to always
process unaligned discards. Previously, we would do this optimization
regardless. This patch changes things to correctly restrict this
optimization to situations where REQ_SECURE exists, but is not included
in the flags.

Signed-off-by: Richard Yao <ryao@gentoo.org>