Linux: Fixes for getrandom fork handling

Linux: Fixes for getrandom fork handling

Careful updates of grnd_alloc.len are required to ensure that
after fork, grnd_alloc.states does not contain entries that
are also encountered by __getrandom_reset_state in TCBs.
For the same reason, it is necessary to overwrite the TCB state
pointer with NULL before updating grnd_alloc.states in
__getrandom_vdso_release.

Before this change, different TCBs could share the same getrandom
state after multi-threaded fork.  This would be a critical security
bug (predictable randomness) if not caught during development.

The additional check in stdlib/tst-arc4random-thread makes it more
likely that the test fails due to the bugs mentioned above.

Both __getrandom_reset_state and __getrandom_vdso_release could
put reserved NULL pointers into the states array.  This is also
fixed with this commit.  After these changes, no null pointers were
observed in the states array during testing.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>