Don't change filp->f_pos in fops->iterate. (linux kernel >= 3.11)

iterate_dir() already change filp->f_pos (using ctx->f_pos).
Thanks to Ivan.

Discussion: https://github.com/psankar/simplefs/commit/3f472cec014f2005bf5fd619ea465592b7ddd879#commitcomment-4057234
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