Fix bitstring allocation on 32-bit platforms

sys/sys/bitstring.h
Fix a rounding calculation that could undersize a bitstring on
32-bit platforms.

tests/sys/sys/bitstring_test.h
Add a test for bitstr_size

PR: 210260
Reported by: Mark Millard
Reviewed by: gibbs
Approved by: re (marius)
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6848