ia64: fix typo in a comment

s/when when/when/

Link: https://lkml.kernel.org/r/20210817112500.12848-1-wangborong@cdjrlc.com
Signed-off-by: Jason Wang <wangborong@cdjrlc.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>