lttng-modules: Backport fixes for kernel instrumentation

lttng-modules: Backport fixes for kernel instrumentation

This backport fixes from upcoming 2.9.4 release. Those are:

 - Fix: vmalloc wrapper on kernel < 2.6.38
 - Fix: vmalloc wrapper on kernel >= 4.12
 - Add kmalloc failover to vmalloc
 - Fix: mmap: caches aliased on virtual addresses
 - Fix: update ext4 instrumentation for kernel 4.13
 - Fix: Sleeping function called from invalid context
 - Fix: sched for v4.11.5-rt1
 - Fix: handle missing ftrace header on v4.12

This fix failures in some BSP layers which are using Linux 4.13
already.

Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Ross Burton <ross.burton@intel.com>