iov_iter: fix copy_page_to_iter_nofault()

The recent conversion to inline functions made two mistakes:

1. It tries to copy the full amount requested (bytes), not just what's
   available in the kmap'd page (n).
2. It's not applying the offset in the first page.

Note that copy_page_to_iter_nofault() is only used by /proc/kcore. This
was detected by drgn's test suite.

Fixes: f198274 ("iov_iter: Convert iterate*() to inline funcs")
Signed-off-by: Omar Sandoval <osandov@fb.com>
Link: https://lore.kernel.org/r/c1616e06b5248013cbbb1881bb4fef85a7a69ccb.1700257019.git.osandov@fb.com
Acked-by: David Howells <dhowells@redhat.com>
Signed-off-by: Christian Brauner <brauner@kernel.org>

lib/iov_iter.c

@@ -409,7 +409,7 @@ size_t copy_page_to_iter_nofault(struct page *page, unsigned offset, size_t byte
 		void *kaddr = kmap_local_page(page);
 		size_t n = min(bytes, (size_t)PAGE_SIZE - offset);
 
-		n = iterate_and_advance(i, bytes, kaddr,
+		n = iterate_and_advance(i, n, kaddr + offset,
 					copy_to_user_iter_nofault,
 					memcpy_to_iter);
 		kunmap_local(kaddr);