From: Hugh Dickins <hugh@veritas.com>

Dissatisfied with earlier fix to race where swapoff sneaks page into tmpfs
page cache after truncate_inode_pages cleaned it: calling it a second time
can be too heavy, instead fix shmem_unuse_inode to check i_size.
(Actually, one part of this fix is in the previous patch: shmem_file_write
now has a hold on the page when it raises i_size.)