Between inode->i_sem and info->lock comes info->sem; but it doesn't
guard thoroughly against the difficult races (truncate during read),
and serializes reads from tmpfs unlike other filesystems.  I'd prefer
to work with just i_sem and info->lock, backtracking when necessary
(when another task allocates block or metablock at the same time).

(I am not satisfied with the locked setting of next_index at the start
of shmem_getpage_locked: it's one lock hold too many, and it doesn't
really fix races against truncate better than before: another patch in
a later batch will resolve that.)