vfs_exports: Tighten bounds and assert consistency of numsecflavors

We know the value must be greater than 0 and less than MAXSECFLAVORS.

Reject values outside this range in the initial check in vfs_export and add KASSERTs
in the later consumers.

Also check that we are called with one of either MNT_DELEXPORT or MNT_EXPORTED set.

Reviewed by: rmacklem
Approved by: mav (mentor)
MFC after: 1 week
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D24753