mount.cifs: fix verbose messages on option parsing

When verbose logging is enabled, invalid credentials file lines may be
dumped to stderr. This may lead to information disclosure in particular
conditions when the credentials file given is sensitive and contains '='
signs.

Bug: https://bugzilla.samba.org/show_bug.cgi?id=15026

Signed-off-by: Jeffrey Bencteux <jbe@improsec.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>