locks: rename lease functions to reflect locks.c conventions

locks: rename lease functions to reflect locks.c conventions

We've been using the convention that vfs_foo is the function that calls
a filesystem-specific foo method if it exists, or falls back on a
generic method if it doesn't; thus vfs_foo is what is called when some
other part of the kernel (normally lockd or nfsd) wants to get a lock,
whereas foo is what filesystems call to use the underlying local
functionality as part of their lock implementation.

So rename setlease to vfs_setlease (which will call a
filesystem-specific setlease after a later patch) and __setlease to
setlease.

Also, vfs_setlease need only be GPL-exported as long as it's only needed
by lockd and nfsd.

Signed-off-by: "J. Bruce Fields" <bfields@citi.umich.edu>