UBIFS: fix races in bit-fields

We cannot store bit-fields together if the processes which
change them may race, unless we serialize them.

Thus, move the nospc and nospc_rp bit-fields eway from
the mount option/constant bit-fields, to avoid races.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>