[PATCH] erofs: allocate extra bvec pages from reserved buffer pool first by default

[Sandeep Dhavale]

> I do not have a good number for it now. While the modification is minor
> and without any side effects. I suggest that we make the change.
>
> Thanks,

Hi Chunhai,
Even though it may not be easily visible in comprehensive tests like
app launch in android, probably it's worth trying with microbenchmark
and profile just this small function/path to quantify the benefits.

Thanks,
Sandeep.