[PATCH] tmpfs 5/7 writepage/truncate race fix

[PATCH] tmpfs 5/7 writepage/truncate race fix

From: Hugh Dickins <hugh@veritas.com>

If it's possible for a tmpfs page beyond i_size to remain in cache until
shmem_truncate repeats truncate_inode_pages, then shmem_writepage's
BUG_ON(index >= info->next_index) cannot be completely safe.  But it's a
useful check in a fragile area, so retain it when not in shmem_truncate.