udev-cache: Don't ignore error messages from cache extract

udev-cache: Don't ignore error messages from cache extract

Previous changes should obviate all known spurious errors coming out of
tar. Since real extraction failures can and will occur, stop redirecting
stdout/stderr to /dev/null.

Take this opportunity to also remove an unnecessary subshell.

Signed-off-by: Richard Tollerton <rich.tollerton@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>