afs: Fix the afs.cell and afs.volume xattr handlers

Fix the ->get handlers for the afs.cell and afs.volume xattrs to pass the
source data size to memcpy() rather than target buffer size.

Overcopying the source data occasionally causes the kernel to oops.

Fixes: d3e3b7eac8 ("afs: Add metadata xattrs")
Signed-off-by: David Howells <dhowells@redhat.com>

fs/afs/xattr.c

@@ -50,7 +50,7 @@ static int afs_xattr_get_cell(const struct xattr_handler *handler,
 		return namelen;
 	if (namelen > size)
 		return -ERANGE;
-	memcpy(buffer, cell->name, size);
+	memcpy(buffer, cell->name, namelen);
 	return namelen;
 }
 
@@ -104,7 +104,7 @@ static int afs_xattr_get_volume(const struct xattr_handler *handler,
 		return namelen;
 	if (namelen > size)
 		return -ERANGE;
-	memcpy(buffer, volname, size);
+	memcpy(buffer, volname, namelen);
 	return namelen;
 }