Show the sctp.checksum_bad filter rather than hiding it.

Follow TCP's lead by putting the filter in a subtree under the checksum.

Change-Id: I9351ee865011cd04bc3d3e88c51e8dbb3dc23f07
Reviewed-on: https://code.wireshark.org/review/4082
Reviewed-by: Jeff Morriss <jeff.morriss.ws@gmail.com>