io.c (fptr_finalize_flush): close race leading to EBADF

The previous ordering was:

a) notify waiting_fd threads of impending close
b) waiting on busy list from a)
c) invalidate fptr->fd
d) calling close()

However, it was possible for a new thread to enter
the waiting_fd list while scheduling on b), leading
to EBADF from those threads when we hit d).

Instead, we now avoid triggering EBADF in other threads by
reordering b) and c)

a) notify waiting_fd threads of impending close
c) invalidate fptr->fd
b) waiting on busy list from a)
d) calling close()

cf. http://ci.rvm.jp/results/trunk-nopara@silicon-docker/1474526

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65937 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

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