From: Hugh Dickins <hugh@veritas.com>

Recent testing has shown that swapoff can sneak a page back into the
tmpfs page cache after truncate_inode_pages has cleaned it, before
shmem_truncate resets next_index to stop that: BUG_ON(inode->i_blocks)
in shmem_delete_inode.  So call truncate_inode_pages again to be safe.