libshare: nfs: commonify nfs_{init,fini}_tmpfile(), nfs_disable_share()

Also open the temp file cloexec

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Wilson <gwilson@delphix.com>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11886