generic ipi function calls: wait on alloc failure fallback

When a GFP_ATOMIC allocation fails, it falls back to allocating the
data on the stack and converting it to a waiting call.

Make sure we actually wait in this case.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>