alpha: potential race around hae_cache in RESTORE_ALL

[Al Viro <viro@ZenIV.linux.org.uk>]

	What happens if we get to RESTORE_ALL with interrupts enabled,
find that we want to restore HAE, get to
        stq     $21, HAE_CACHE($19);    \
and get hit by an interrupt right after that assignment?  Note that
*alpha_mv->hae_register is still not updated, but alpha_mv->hae_cache
already is, so if the interrupt calls set_hae() it would get seriously
confused if the value it wants is equal to the value we've put into
->hae_cache.

	Until ~2002 it used to have a couple of swpipl around these
assignments and __set_hae() is still doing those.  I agree that on
many exits we *will* have interrupts disabled when we get to RESTORE_ALL,
but not on all of them.  E.g. any interrupt taken in kernel mode will
happily go to restore_all without bothering with swpipl at all.

	AFAICS, it looks like a race; the change in question had been
introduced in "Update Alpha UP for thread_info and scheduler changes"
(Feb 10 2002, commit 374eeee8a8a50e12278dfa37021df7b6efe506c3 in historical
git tree).

	Comments?