o Remove last argument to ip_fragment(), and obtain all needed information

o Remove last argument to ip_fragment(), and obtain all needed information
  on checksums directly from mbuf flags. This simplifies code.
o Clear CSUM_IP from the mbuf in ip_fragment() if we did checksums in
  hardware. Some driver may not announce CSUM_IP in theur if_hwassist,
  although try to do checksums if CSUM_IP set on mbuf. Example is em(4).
o While here, consistently use CSUM_IP instead of its alias CSUM_DELAY_IP.
  After this change CSUM_DELAY_IP vanishes from the stack.

Submitted by:	Sebastian Kuzminsky <seb lineratesystems.com>