Retry export if getfh fails.

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>