BUG/MINOR: threads: fix the process range of thread masks

BUG/MINOR: threads: fix the process range of thread masks

Commit 421f02e ("MINOR: threads: add a MAX_THREADS define instead of
LONGBITS") used a MAX_THREADS macros to fix threads limits. However,
one change was wrong as it affected the upper bound of the process
loop when setting threads masks. No backport is needed.