Use f_bsize for ZFS afs_fsfragsize

Use f_bsize for ZFS afs_fsfragsize

On ZFS, the disk space files can use up can be rounded up to the next
recordsize boundary if they've been truncated. This can cause the Unix CM
to mis-estimate cache usage, since it truncates files fairly often, and
assumes the disk space used is the file length rounded up to the next
f_frsize.

Since the ZFS recordsize is available via the statvfs f_bsize, just
round up to that instead. There is still some additional file metadata
that takes up some additional space on disk, but according to ZFS people
I've spoken to about this, it cannot be known in advance. In practice,
the additional metadata storage doesn't appear to exceed about 10% of
the data storage, which should be acceptable.

FIXES 125365

Reviewed-on: http://gerrit.openafs.org/650
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Tested-by: Andrew Deason <adeason@sinenomine.net>