[MSF] Fix FPM interval calcluation

We have some code to try to determine how many pieces an MSF
Free Page Map is split into, and this code had an off by one
error which would cause the calculation to be incorrect when
there were exactly 4096*k + 1 blocks in an MSF file.

Original investigation and patch outline by Colden Cullen.

Differential Revision: https://reviews.llvm.org/D41742

llvm-svn: 321880