Clean out-of-date function ``d_alloc_root''

Howdy Agriel,
     I forked your repo for learning how to write a real filesystem. I found the following function is out-of-date. And it will give me an error when I try to compile it. here is me commit: 
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``d_alloc_root'' is out of date. And it will be a compiling-time error now.
Here is the doc from kernel doc:
        d_alloc_root() is gone, along with a lot of bugs caused by code
misusing it.  Replacement: d_make_root(inode).  The difference is,
d_make_root() drops the reference to inode if dentry allocation fails.

So i change it to ``d_make_root''.
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will you please merge my patch?
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Thanks,
Madper Xie.