[CRYPTO] api: scatterwalk_copychunks() fails to advance through scatterlist

In the loop in scatterwalk_copychunks(), if walk->offset is zero,
then scatterwalk_pagedone rounds that up to the nearest page boundary:

		walk->offset += PAGE_SIZE - 1;
		walk->offset &= PAGE_MASK;

which is a no-op in this case, so we don't advance to the next element
of the scatterlist array:

		if (walk->offset >= walk->sg->offset + walk->sg->length)
			scatterwalk_start(walk, sg_next(walk->sg));

and we end up copying the same data twice.

It appears that other callers of scatterwalk_{page}done first advance
walk->offset, so I believe that's the correct thing to do here.

This caused a bug in NFS when run with krb5p security, which would
cause some writes to fail with permissions errors--for example, writes
of less than 8 bytes (the des blocksize) at the start of a file.

A git-bisect shows the bug was originally introduced by
5c64097a, first in 2.6.19-rc1.
Signed-off-by: N"J. Bruce Fields" <bfields@citi.umich.edu>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>

crypto/scatterwalk.c

@@ -91,6 +91,8 @@ void scatterwalk_copychunks(void *buf, struct scatter_walk *walk,
 		memcpy_dir(buf, vaddr, len_this_page, out);
 		scatterwalk_unmap(vaddr, out);
 
+		scatterwalk_advance(walk, nbytes);
+
 		if (nbytes == len_this_page)
 			break;
 
@@ -99,7 +101,5 @@ void scatterwalk_copychunks(void *buf, struct scatter_walk *walk,
 
 		scatterwalk_pagedone(walk, out, 1);
 	}
-
-	scatterwalk_advance(walk, nbytes);
 }
 EXPORT_SYMBOL_GPL(scatterwalk_copychunks);