posixshm: fix setting of shm_flags

[kevans]

Summary

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".

[kevans]

(I won't be jumping to commit anything to avoid creating another merge conflict for the non-transparent super page patch)

[kib]

This is fine, and it does not conflict with large pages patch.
More, there is a reason why I prefer to get this in before the final testing for large pages.