r21324: Add linux setlease to the vfs layer. Next round, as Volker points out,

r21324: Add linux setlease to the vfs layer.  Next round, as Volker points out,
it should be abstracted a little higher up so other os'es can have an
entry, but it will take a bit more work.  Thanks to Chetan Shringarpure
and Mathias Dietz.

I didn't increment the vfs number again because the kernel change notify
stuff hasn't been released yet anyway.