cxgbe/t4_tom: adjust the hardware receive window to match changes to the
receive sockbuf's high water mark.
Calculate rx credits on the spot instead of tracking sbused/sb_cc and
rx_credits in the toepcb. The previous method worked when the high
water mark changed due to SB_AUTOSIZE but not when it was adjusted
directly (for example, by the soreserve in nfsrvd_addsock).
This fixes a connection hang while running iozone over an NFS mounted
share where nfsd's TCP sockets are being handled by t4_tom.
Sponsored by: Chelsio Communications