cifs: convert prefixpath delimiters in cifs_build_path_to_root

Regression from 2.6.39...

The delimiters in the prefixpath are not being converted based on
whether posix paths are in effect. Fixes:

    https://bugzilla.redhat.com/show_bug.cgi?id=727834

Reported-and-Tested-by: Iain Arnell <iarnell@gmail.com>
Reported-by: Patrick Oltmann <patrick.oltmann@gmx.net>
Cc: Pavel Shilovsky <piastryyy@gmail.com>
Cc: stable@kernel.org
Signed-off-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>