Clean up DTLS1_BITMAP code.

Take the sequence number as a parameter. Also replace satsub64be with
the boring thing: convert to uint64_t and subtract normally.

BUG=468889

Change-Id: Icab75f872b5e55cf4e9d68b66934ec91afeb198b
Reviewed-on: https://boringssl-review.googlesource.com/5558
Reviewed-by: Adam Langley <agl@google.com>