tracing: Free saved_tgids memory in free_saved_cmdlines_buffer

saved_tgids memory was not freed in free_saved_cmdlines_buffer
which can cause memory-leak when /d/tracing/saved_cmdlines_size
node value is changed. Fix it by freeing saved_tgids
in free_saved_cmdlines_buffer.

Change-Id: I006870c858b7306b7d9c840b7712061985dda310
Signed-off-by: Omprakash Dhyade <odhyade@codeaurora.org>