Close a race between libzfs and mountd when updating NFS exports.

- Flush the file descriptor for the new ZFS exports file before
  sending a SIGHUP to mountd.

Reviewed by:	pjd
Approved by:	ken
MFC after:	3 days

cddl/compat/opensolaris/misc/fsshare.c

@@ -223,6 +223,7 @@ fsshare_main(const char *file, const char *mountpoint, const char *shareopts,
 			error = errno;
 			unlink(tmpfile);
 		} else {
+			fflush(newfd);
 			/*
 			 * Send SIGHUP to mountd, but unlock exports file later.
 			 */