tmpfs 1/5 swapoff deadlock: my igrab/iput around the yield in
shmem_unuse_inode was rubbish, seems my testing never really hit the
case until last week, when truncation of course deadlocked on the page
held locked across the iput (at least I had the foresight to say "ugh!"
there).  Don't yield here, switch over to the simple backoff I'd been
using for months in the loopable tmpfs patch (yes, it could loop
indefinitely for memory, that's already an issue to be dealt with
later).  The return convention from shmem_unuse to try_to_unuse is
inelegant (commented at both ends), but effective.