Staging: lustre: module: Replace function calls

Replace the calls of function cfs_trace_free_string_buffer() with
kfree() as the former function is not required.

Signed-off-by: Shivani Bhardwaj <shivanib134@gmail.com>
Acked-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>