Address two incorrect calculations and enhance readability of PRR code

• address second instance of cwnd potentially becoming zero
• fix sublte bug due to implicit int to uint typecase in max()
• fix bug due to typo in hand-coded CEILING() function by using howmany() macro
• use int instead of long, and add a missing long typecast
• replace if conditionals with easier to read imax/imin (as in pseudocode)

Reviewed By: transport, kbowling
MFC after: 3 days
Sponsored by: NetApp, Inc.
Differential Revision: https://reviews.freebsd.org/D28813