fs-verity: use kmap_local_page() instead of kmap()

Convert the use of kmap() to its recommended replacement
kmap_local_page().  This avoids the overhead of doing a non-local
mapping, which is unnecessary in this case.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Fabio M. De Francesco <fmdefrancesco@gmail.com>
Link: https://lore.kernel.org/r/20220818224010.43778-1-ebiggers@kernel.org

fs/verity/read_metadata.c

@@ -53,14 +53,14 @@ static int fsverity_read_merkle_tree(struct inode *inode,
 			break;
 		}
 
-		virt = kmap(page);
+		virt = kmap_local_page(page);
 		if (copy_to_user(buf, virt + offs_in_page, bytes_to_copy)) {
-			kunmap(page);
+			kunmap_local(virt);
 			put_page(page);
 			err = -EFAULT;
 			break;
 		}
-		kunmap(page);
+		kunmap_local(virt);
 		put_page(page);
 
 		retval += bytes_to_copy;