(CFACT-131) Fix block size calculation for mountpoints fact.

Currently the optimal transfer block size member (f_bsize) was
being used in size calculations instead of the fragment
size member (f_frsize) on Linux and Solaris.

The fix is to use f_frsize instead of f_bsize where appropriate.

Note: on BSD, the statfs struct has f_bsize as the block size,
as anyone sane would expect.