Refactoring: syscall tracing: combine common fields into struct lttng_kernel_syscall_...

Refactoring: syscall tracing: combine common fields into struct lttng_kernel_syscall_table

There was a lot of code duplication between syscall tables associated
with ring buffers and those associated with event notifiers.

Move common fields into a struct lttng_kernel_syscall_table, and add a
syscall_table field in both struct lttng_event_notifier_group and
struct lttng_kernel_channel_common_private.

This allows removing both syscall_entry_event_notifier_probe and
syscall_exit_event_notifier_probe by using the syscall_entry_event_probe
and syscall_exit_event_probe instead for event notifiers as well.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: Ifb78755429ebf8699ac4bd3bb66af16bda1d86c4