If find_writable_file() returns null, cifs_file_flush will return 0 without issuing set_file_size, and the outer 'if (!rc)' block will set i_size to 0 before telling the server to truncate. If the cifs_open() then fails, the inode will have size 0, while the server file is unchanged.
Move the netfs_resize_file() and cifs_setsize() into the 'if (cfile)', so they only run after a successful set_file_size.
In the no-handle else branch, evict stale pages with truncate_inode_pages before the O_TRUNC open to dispose of old cache pages, and let the open response set the i_size.