vfs lease api

["J. Bruce Fields" <bfields@fieldses.org>]

Eventually we want to be able to support NFSv4 delegations for cluster
filesystem exports.  We implement NFSv4 delegations using leases.  So to
make this work, we need leases to be passed down to the filesystem, so
that a cluster filesystem can enforce leases correctly across all nodes.

The following patches do some minor cleanup of the lease code, and then
add a setlease() file method.

For now the only implementations of setlease() we include (for NFS and
GFS2) are used only to turn off leases selectively.  To complete this
work we'll need to implement proper lease support for GFS2 or OCFS2, and
probably do some more work on lease breaking.

But we're requesting this be included in -mm now in the theory that the
lease-disabling behavior is useful now anyway, and in hopes that this
will the required further work.

I'm also keeping this in the "for-mm" branch at:

	git://linux-nfs.org/~bfields/linux.git for-mm 

(And maybe you could also remove the server-cluster-locking-api branch
from your list of git fetches, if it's still there?--I don't intend to
use it any more.)

--b.