OS-4319 zfs mishandles partial writes

*** 663,672 ****
--- 663,673 ----
          zilog_t         *zilog;
          offset_t        woff;
          ssize_t         n, nbytes;
          int             max_blksz = zfsvfs->z_max_blksz;
          int             error = 0;
+         int             prev_error;
          arc_buf_t       *abuf;
          iovec_t         *aiov = NULL;
          xuio_t          *xuio = NULL;
          int             i_iov = 0;
          int             iovcnt = uio->uio_iovcnt;
*** 970,995 ****
                   * account for possible concurrent updates.
                   */
                  while ((end_size = zp->z_size) < uio->uio_loffset) {
                          (void) atomic_cas_64(&zp->z_size, end_size,
                              uio->uio_loffset);
-                         ASSERT(error == 0);
                  }
                  /*
                   * If we are replaying and eof is non zero then force
                   * the file size to the specified eof. Note, there's no
                   * concurrency during replay.
                   */
                  if (zfsvfs->z_replay && zfsvfs->z_replay_eof != 0)
                          zp->z_size = zfsvfs->z_replay_eof;
  
                  error = sa_bulk_update(zp->z_sa_hdl, bulk, count, tx);
  
                  zfs_log_write(zilog, tx, TX_WRITE, zp, woff, tx_bytes, ioflag);
                  dmu_tx_commit(tx);
  
!                 if (error != 0)
                          break;
                  ASSERT(tx_bytes == nbytes);
                  n -= nbytes;
  
                  if (!xuio && n > 0)
--- 971,1000 ----
                   * account for possible concurrent updates.
                   */
                  while ((end_size = zp->z_size) < uio->uio_loffset) {
                          (void) atomic_cas_64(&zp->z_size, end_size,
                              uio->uio_loffset);
                  }
                  /*
                   * If we are replaying and eof is non zero then force
                   * the file size to the specified eof. Note, there's no
                   * concurrency during replay.
                   */
                  if (zfsvfs->z_replay && zfsvfs->z_replay_eof != 0)
                          zp->z_size = zfsvfs->z_replay_eof;
  
+                 /*
+                  * Keep track of a possible pre-existing error from a partial
+                  * write via dmu_write_uio_dbuf above.
+                  */
+                 prev_error = error;
                  error = sa_bulk_update(zp->z_sa_hdl, bulk, count, tx);
  
                  zfs_log_write(zilog, tx, TX_WRITE, zp, woff, tx_bytes, ioflag);
                  dmu_tx_commit(tx);
  
!                 if (prev_error != 0 || error != 0)
                          break;
                  ASSERT(tx_bytes == nbytes);
                  n -= nbytes;
  
                  if (!xuio && n > 0)