Btrfs: add support for fallocate's zero range operation

This implements support the zero range operation of fallocate. For now
at least it's as simple as possible while reusing most of the existing
fallocate and hole punching infrastructure.
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>