switch_root: code shrink - use iterate_on_dir()

function                                             old     new   delta
rmrf                                                   -      46     +46
delete_contents                                      181      99     -82
------------------------------------------------------------------------------
(add/remove: 1/0 grow/shrink: 0/1 up/down: 46/-82)            Total: -36 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>
1 file changed