ucm: confine sysw and cfg-save paths with realpath()

ucm: confine sysw and cfg-save paths with realpath()

1: execute_sysw() concatenated the sysfs root with a caller path via
snprintf() with no traversal check. Use realpath() and reject paths
that escape the sysfs root.

2: execute_cfgsave() opened a caller-supplied filename with no
restriction. Confine writes to XDG_RUNTIME_DIR (or /tmp) via
realpath().

[ rewritten with check_realpath() helper function -- tiwai ]

Closes: https://github.com/alsa-project/alsa-lib/pull/527
Signed-off-by: Harsh Raj Singhania <raj.harshraut@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>