fbdev: modedb: fix a possible UAF in fb_find_mode()

If mode_option is NULL, it is assigned from mode_option_buf:

  if (!mode_option) {
    fb_get_options(NULL, &mode_option_buf);
    mode_option = mode_option_buf;
  }

Later, name is assigned from mode_option:

  const char *name = mode_option;

However, mode_option_buf is freed before name is no longer used:

  kfree(mode_option_buf);

while name is still accessed by:

  if ((name_matches(db[i], name, namelen) ||

Since name aliases mode_option_buf, this may result in a
use-after-free.

Fix this by extending the lifetime of mode_option_buf until the end of the
function by using scope-based resource management for cleanup.

Signed-off-by: Tuo Li <islituo@gmail.com>
Cc: stable@vger.kernel.org # v6.5+
Signed-off-by: Helge Deller <deller@gmx.de>