mountd: prefer explicit subexports over crossmnt parents

If a parent is exported with crossmnt, and if a child is also explicitly
exported, then both exports could potentially produce matches in this
loop; that isn't a bug.

Instead of warning and ignoring the second match we find, we should
instead prefer whichever export is deeper in the tree, so that
children's options can override those of their parents.

Reported-by: Olga Kornievskaia <aglo@citi.umich.edu>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>