cifs: fix handling of escaped ',' in the password mount argument

Passwords can contain ',' which are also used as the separator between
mount options. Mount.cifs will escape all ',' characters as the string ",,".
Update parsing of the mount options to detect ",," and treat it as a single
'c' character.

Fixes: 24e0a1ef ("cifs: switch to new mount api")
Cc: stable@vger.kernel.org # 5.11
Reported-by: Simon Taylor <simon@simon-taylor.me.uk>
Tested-by: Simon Taylor <simon@simon-taylor.me.uk>
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>