Changed FromStatfsT on Darwin to fix df output

[bolshevik]

osxfuse/fuse provides a possibility to implement a standard and an extended statfs calls, the previous mapping is relevant for the statfs_x call, which is not implemented in go-fuse yet.

https://github.com/osxfuse/fuse/blob/1cd72d7333ce84975470f5d865383422034baa7d/include/fuse.h#L620
https://github.com/osxfuse/fuse/blob/abc35f85fdede186ed164082bd71fc1217d2f58f/lib/fuse.c#L4094

This change maps the Block Size to both upper-level block size and fragment size.

Previously it resulted into incorrect reporting of total and free space in the "df" command.

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[hanwen]

this needs a CLA.

the comments on the code you change suggest that I have actually thought about this when I wrote this, and that it wasn't a random oversight.

Could you add a test that exercises statfs on OSX that exercises this new behavior?

[hanwen]

no reponse for 6 months; closing.