vfs_export: Simplify vfs_export_lookup

If the filesystem is not exported directly return NULL.
If no address is given and filesystem is exported using some default
one return it directly, if it doesn't have a default one directly
return NULL.

Reviewed by: kib, bapt
MFC after: 1 week
Sponsored by: Gandi.net
Differential Revision: https://reviews.freebsd.org/D12505