Enable constructing a Prependable from a View without allocating.

PiperOrigin-RevId: 211722525
Change-Id: Ie73753fd09d67d6a2ce70cfe2d4ecf7275f09ce0

pkg/tcpip/buffer/prependable.go

@@ -32,6 +32,15 @@ func NewPrependable(size int) Prependable {
 	return Prependable{buf: NewView(size), usedIdx: size}
 }
 
+// NewPrependableFromView creates an entirely-used Prependable from a View.
+//
+// NewPrependableFromView takes ownership of v. Note that since the entire
+// prependable is used, further attempts to call Prepend will note that size >
+// p.usedIdx and return nil.
+func NewPrependableFromView(v View) Prependable {
+	return Prependable{buf: v, usedIdx: 0}
+}
+
 // Prepend reserves the requested space in front of the buffer, returning a
 // slice that represents the reserved space.
 func (p *Prependable) Prepend(size int) []byte {