Clean-up shm directory tree after freeing the channel

On NFS, an unlinked file that is still mmaped or unclosed is kept
around by creating a .nfsXXXX file since it may still be in use. [1]

This prevents the effective cleanup of the shm tree directory because
it happens before the userspace consumer shm handles table
cleanup. [2]

Moving the tree removal to lttng_ustconsumer_free_channel ensures that
the cleanup is done when the files are completely closed/unmapped.

[1] http://nfs.sourceforge.net/ Look for "silly rename"
[2] See channel_free subcall to shm_object_table_destroy

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>