efivarfs: use I_MUTEX_CHILD nested lock to traverse variables on resume

syzbot warns about a potential deadlock, but this is a false positive
resulting from a missing lockdep annotation: iterate_dir() locks the
parent whereas the inode_lock() it warns about locks the child, which is
guaranteed to be a different lock.

So use inode_lock_nested() instead with the appropriate lock class.

Reported-by:  <syzbot+019072ad24ab1d948228@syzkaller.appspotmail.com>
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>