nullfs: stop lying about mount flags in statfs(2)

Flags should not propagate from the lower fs. Behavior for the upper fs
is determined by flags from its mount point structure. When lower fs
acts according to its mount configuration, it is reported up as VOP
errors.

PR: 283425
Reviewed by: markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D48150