CVE-2026-72217: SUNRPC: Bound-check xdr_buf_to_bvec() stores before writing

In the Linux kernel, the following vulnerability has been resolved:

SUNRPC: Bound-check xdrbuftobvec() stores before writing

xdrbuftobvec() writes a biovec into the caller's array before testing whether that slot is in range, and the head branch performs the store with no check at all. When the caller's budget is exactly used up, the next store lands one element past the end of the array. The overflow label returns count - 1, which masks the surplus store but cannot undo it.

rqbvec, the array passed by nfsdvfswrite(), is allocated to exactly rqmaxpages entries with no slack. The OOB store can land in adjacent slab memory; the bvlen and bvoffset fields written there are derived from client-supplied RPC payload sizes.

Move the in-range check ahead of the store in the head, page-loop, and tail branches. With the check at the top of each sequence, count is incremented only after a successful store, so the overflow label can return count directly.