Fix: truncate the metadata file in shm-path

Fix: truncate the metadata file in shm-path

In the shm-path mode, the metadata will be backuped to a metadata
file, when run the lttng command "lttng metadata regenerate" to
resample the wall time following a major NTP correction, the metadata
file will not be truncated and regenerated.

Add the function clear_metadata_file() to truncate and regenerate the
metadata file.

Signed-off-by: Liguang Li <liguang.li@windriver.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>