Use d_alloc_root() instead of d_make_root() in linux kernel < 3.3

d_alloc_root() is gone in 32991ab305ace7017c62f8eecbe5eb36dc32e13b.

Link: https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=32991ab305ace7017c62f8eecbe5eb36dc32e13b