Fix: update block instrumentation to compile on 4.8 kernels

Fix: update block instrumentation to compile on 4.8 kernels

These 2 upstream Linux commits break the block instrumentation in lttng
modules master:

https://github.com/torvalds/linux/commit/4e1b2d52a80d79296a5d899d73249748dea71a53

https://github.com/torvalds/linux/commit/288dab8a35a0bde426a09870943c8d3ee3a50dab

The errors are :

lttng-modules/probes/../instrumentation/events/lttng-module/block.h:35:13:
error: ‘REQ_DISCARD’ undeclared (first use in this function)

lttng-modules/probes/../instrumentation/events/lttng-module/block.h:41:14:
error: ‘REQ_SECURE’ undeclared (first use in this function)

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>