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>