src: Fix an error for the loop initialization declaration

src: Fix an error for the loop initialization declaration

When compile them old gcc, get the following error
mount-idmapped.c: In function 'parse_map':
mount-idmapped.c:154:2: error: 'for' loop initial declarations are only allowed in C99 mode
  for (int i = 0; i < 2; i++) {

Fix them by declaring them at the beggining instead of in loop.

Signed-off-by: Yang Xu <xuyang2018.jy@fujitsu.com>
Reviewed-by: Christian Brauner <christian.brauner@ubuntu.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>