grace period setting

["J. Bruce Fields" <bfields@citi.umich.edu>]

The NFSv4 lease-time configuration interface we currently have doesn't
actually allow setting the grace period directly.

Mostly this is noticed by developers trying to set a smaller grace
period to speed testing.

However, it also makes it difficult to make reboot-recovery behavior
correct, since there's no way for userland to force the kernel to
observe the correct grace period on reboot if the previous server was
using something other than the default lease time.

So, add a grace-period setting interface as well.

Note if you actually want a shorter grace period you'll also need to
adjust the nlm_grace_period sysctl (/proc/sys/fs/nfs/nlm_grace_period),
since the server doesn't end grace until both have expired.

--b.