cifs: fix mounts to subdirectories of target

The "prefixpath" mount option needs to be ignored
which was missed in the recent conversion to the
new mount API (prefixpath would be set by the mount
helper if mounting a subdirectory of the root of a
share e.g. //server/share/subdir)

Fixes: 24e0a1ef

 ("cifs: switch to new mount api")
Suggested-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
Reviewed-by: Ronnie Sahlberg <lsahlber@redhat.com>