[MEDIUM] session: don't resync FSMs on non-interesting changes

[MEDIUM] session: don't resync FSMs on non-interesting changes

While processing the session, we used to resync the FSMs when buffer
flags changed. But since BF_KERN_SPLICING and BF_READ_DONTWAIT were
introduced, sometimes we could resync after they were set, which is
not what we want. This was because there were some old checks left
which did not mask changes with BF_MASK_STATIC before checking.