pNFS/flexfiles: fix incorrect size check in decode_nfs_fh()

We (adam zabrocki, alexander matrosov, alexander tereshkin, maksym
bazalii) observed the check:

	if (fh->size > sizeof(struct nfs_fh))

should not use the size of the nfs_fh struct which includes an extra two
bytes from the size field.

struct nfs_fh {
	unsigned short         size;
	unsigned char          data[NFS_MAXFHSIZE];
}

but should determine the size from data[NFS_MAXFHSIZE] so the memcpy
will not write 2 bytes beyond destination.  The proposed fix is to
compare against the NFS_MAXFHSIZE directly, as is done elsewhere in fs
code base.

Fixes: d67ae825 ("pnfs/flexfiles: Add the FlexFile Layout Driver")
Signed-off-by: NNikola Livic <nlivic@gmail.com>
Signed-off-by: NDan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: NTrond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/flexfilelayout/flexfilelayout.c

@@ -106,7 +106,7 @@ static int decode_nfs_fh(struct xdr_stream *xdr, struct nfs_fh *fh)
 	if (unlikely(!p))
 		return -ENOBUFS;
 	fh->size = be32_to_cpup(p++);
-	if (fh->size > sizeof(struct nfs_fh)) {
+	if (fh->size > NFS_MAXFHSIZE) {
 		printk(KERN_ERR "NFS flexfiles: Too big fh received %d\n",
 		       fh->size);
 		return -EOVERFLOW;