sdcardfs: change d_u.d_child to d_child

d_u.d_child was switched to just d_child in commit "move d_rcu from overlapping d_child to overlapping d_alias"

Change-Id: I18d6f2fcc0bb93092e82a50dc11193115298ba45
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>