Address two incorrect calculations and enhance readability of PRR code

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