aufs: bugfix, umount passes NULL to ->parse_monolithic()

commit 25f5c5fdcb1b6c2a47f58b59c74547538b7b7c2a aufs5-standalone upstream.

The parameter "void *data" is NULL in unmounting.

Reported-by: PB on github
Signed-off-by: J. R. Okajima <hooanon05g@gmail.com>
See-also: https://github.com/sfjro/aufs5-standalone/issues/9
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>