prison_check_nfsd: Add check for enforce_statfs != 0

Since mountd(8) will not be able to do exports
when running in a vnet prison if enforce_statfs is
set to 0, add a check for this to prison_check_nfsd().

Reviewed by: jamie, markj
MFC after: 2 months
Differential Revision: https://reviews.freebsd.org/D38189