[$] Supporting untorn buffered writes

At last year'sLinux Storage, Filesystem,Memory-Management, and BPF Summit (LSFMM+BPF), there was a discussion about atomic writes that wasaccompanied by patches to support the feature in the block layer, and fordirect I/O on XFS. Thatwork was merged, but another piece of that discussion concerned adding thefeature for buffered I/O, in part because the PostgreSQL database currentlyhas to jump through hoops to ensure that its writes are not "torn"(partially written) when there is an error or crash. Luis Chamberlain leda combined storage and filesystem track at this year's summit to revisitthe idea of providing atomic (or untorn) writes for buffered I/O.