Fix: lttng_this_cpu_ptr wrapper for kernel 3.19+

Starting from kernel 3.19-rc1, __get_cpu_var() disappears, replaced by
this_cpu_ptr(). Introduce a wrapper using either depending on the kernel
version. Older kernels did not expose a this_cpu_ptr(), so keep on using
__get_cpu_var() on old kernels.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Fix: introduce lttng_get_unused_fd() wrapper for 3.19 kernels

get_unused_fd disappears in 3.19 kernel, where get_unused_fd_flags(0)
should be used instead.

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