Noted in D24652, we currently set shmfd->shm_flags on every shm_open()/shm_open2(). This wasn't properly thought out; one shouldn't be able to specify incompatible flags on subsequent opens of non-anon shm.
Move setting of shm_flags explicitly to the two places shmfd are created, as we do with seals, and validate when we're opening a pre-existing mapping that we've either passed no flags or we've passed the exact same flags as the last time.
I'm unsure if it makes more sense to drop the 'shmfd->shm_flags != 0` condition and forcing the exact same flags on subsequent opens. That seems like it'd make it a little harder to use, and in theory specifying shmflags = 0 would be a sensible convention for "whatever was already established".