nfsd: Make loop calling VOP_ALLOCATE() iterate until done

The NFSv4.2 Deallocate operation loops on VOP_DEALLOCATE()
while progress is being made (remaining length decreasing).
This patch changes the loop on VOP_ALLOCATE() for the NFSv4.2
Allocate operation do the same, instead of stopping after
an arbitrary 20 iterations.

(cherry picked from commit 13914e51eb8de6fe9f627c9c1d48c09880b2607e)