libtracefs: Unmap mmap mapping on tracefs_cpu close

The tracefs_cpu_open_mapped() will mmap the ring buffer if it is supported,
but it does not unmap it when it is closed.

Link: https://lore.kernel.org/linux-trace-devel/20240109205112.74225-2-rostedt@goodmis.org

Cc: Vincent Donnefort <vdonnefort@google.com>
Fixes: 2ed14b59 ("libtracefs: Add ring buffer memory mapping APIs")
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>