This is Debian bug #584104, which we tagged as release critical:
As of version 2:4.5-1, mount.cifs appears to ignore the presence of the
'credentials' option given in the command line:
sham@sham-desktop:/tmp$ sudo mount.cifs -o credentials=/etc/auto.smb.gibson
This ends up impacting on autofs as well, as smb/cifs shares are no longer
I can confirm that downgrading cifs-utils and smbfs to the previous package
version (2:4.1-1) makes this problem go away.
Patch for this is already in git:
It should make the 4.6 release.