Fix: lttng_kvmalloc helper NULL pointer OOPS

The static function __vmalloc_node is not visible by KALLSYMS_ALL on at
least some kernels, which leads to a call to a NULL function when trying
to perform allocation of lttng buffer memory under memory fragmentation
conditions (kmalloc_node failure).

Use __vmalloc_node_range instead, and check that the returned pointer
is non-NULL to ensure this type of failure does not happen in any
condition.

Fallback to __vmalloc(), even though it is not NUMA-aware, in case
we fail to find __vmalloc_node_range, and print an explicit warning
to the user console about the need to enable KALLSYMS_ALL.

This affects kernels < 4.12. Later kernels provide kvmalloc(), which
we use.

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