Cherry-picked upstream out-of-order emubd testing

More information upstream (f2a6f45, fc2aa33, 7873d81), but this adds
LFS_EMUBD_POWERLOS_OOO for testing out-of-order block devices that
require sync to be called for things to serialize. It's a simple
implementation, just reverts the first write since last sync on
powerloss, but gets the job done.

Cherry-picking these changes required reverting emubd's scratch buffer,
but carrying around an extra ~block_size of memory isn't a big deal
here.

bd/lfs_emubd.h

@@ -36,17 +36,18 @@ extern "C"
 // Not that read-noop is not allowed. Read _must_ return a consistent (but
 // may be arbitrary) value on every read.
 typedef enum lfs_emubd_badblock_behavior {
-    LFS_EMUBD_BADBLOCK_PROGERROR,
-    LFS_EMUBD_BADBLOCK_ERASEERROR,
-    LFS_EMUBD_BADBLOCK_READERROR,
-    LFS_EMUBD_BADBLOCK_PROGNOOP,
-    LFS_EMUBD_BADBLOCK_ERASENOOP,
+    LFS_EMUBD_BADBLOCK_PROGERROR  = 0, // Error on prog
+    LFS_EMUBD_BADBLOCK_ERASEERROR = 1, // Error on erase
+    LFS_EMUBD_BADBLOCK_READERROR  = 2, // Error on read
+    LFS_EMUBD_BADBLOCK_PROGNOOP   = 3, // Prog does nothing silently
+    LFS_EMUBD_BADBLOCK_ERASENOOP  = 4, // Erase does nothing silently
 } lfs_emubd_badblock_behavior_t;
 
 // Mode determining how power-loss behaves during testing. For now this
 // only supports a noop behavior, leaving the data on-disk untouched.
 typedef enum lfs_emubd_powerloss_behavior {
-    LFS_EMUBD_POWERLOSS_NOOP,
+    LFS_EMUBD_POWERLOSS_NOOP = 0, // Progs are atomic
+    LFS_EMUBD_POWERLOSS_OOO  = 1, // Blocks are written out-of-order
 } lfs_emubd_powerloss_behavior_t;
 
 // Type for measuring read/program/erase operations
@@ -126,6 +127,7 @@ typedef struct lfs_emubd_block {
 typedef struct lfs_emubd_disk {
     uint32_t rc;
     int fd;
+    uint8_t *scratch;
 } lfs_emubd_disk_t;
 
 // emubd state
@@ -138,6 +140,8 @@ typedef struct lfs_emubd {
     lfs_emubd_io_t proged;
     lfs_emubd_io_t erased;
     lfs_emubd_powercycles_t power_cycles;
+    lfs_ssize_t ooo_block;
+    lfs_emubd_block_t *ooo_data;
     lfs_emubd_disk_t *disk;
 
     const struct lfs_emubd_config *cfg;