cifs: Improve guard for excluding $LXDEV xattr

$LXDEV xattr is for storing block/char device's major and minor number.
Change guard which excludes storing $LXDEV xattr to explicitly filter
everything except block and char device. Current guard is opposite, which
is currently correct but is less-safe. This change is required for adding
support for creating WSL-style symlinks as symlinks also do not use
device's major and minor numbers.

Signed-off-by: Pali Rohár <pali@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>