Retry export if getfh fails.

mountd tries to avoid telling the kernel to export something
when the kernel already knows to do that.
However sometimes (exportfs -r) the kernel can be told
to forget something without mountd realising.
So if mountd finds that it cannot get a  valid filehandle,
make sure it really has been exported to the kernel.

This only applies if the nfsd filesystem is not mounted.

Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Steve Dickson <steved@redhat.com>