UBIFS: fix budget leak in error path

We forgot to free the budget in 'write_begin_slow()' when 'do_readpage()'
fails. This patch fixes the issue.

Suggested-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>

fs/ubifs/file.c

@@ -262,6 +262,7 @@ static int write_begin_slow(struct address_space *mapping,
 			if (err) {
 				unlock_page(page);
 				page_cache_release(page);
+				ubifs_release_budget(c, &req);
 				return err;
 			}
 		}