mountd: fix export upcall failure in use_ipaddr case

mountd: fix export upcall failure in use_ipaddr case

After 0509d3428f523 "mountd: Replace "struct hostent" with "struct
addinfo"", the export upcall fails in the use_ipaddr case.

I think we never noticed because a) the use_ipaddr case is rarer than
the !use_ipaddr case, and b) the nfsd_fh upcall does a preemptive export
downcall that renders the nfsd export call unnecessary in some cases.

Cc: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>
Signed-off-by: Steve Dickson <steved@redhat.com>