Fixed another subtle corner cases with noop sync broadcasting

This is an extension of the noop-sync after unrelated write-sync after
desync corner case:

  op                 a state         b state
                     in-sync         in-sync
  desync(b)          in-sync         desync
  write(a)           unsync          desync
  sync(a)            in-sync'        desync
  sync(b)            in-sync         in-sync

But instead of explicitly calling lfsr_file_sync, what if you implicitly
triggered sync through something like a write on a file with the
LFS_O_SYNC flag, but not a normal write, a noop write, write(0)?

If the definition of LFS_O_SYNC is taken literally as "lfsr_file_write
and friends implicitly call lfsr_file_sync after every call", then this
should behave just as if lfsr_file_sync had been called, and
unconditionally broadcast the sync. Since this is the simplest
interpretation, I think this is what we should implement.

Added tests, and adopted this behavior. Fortunately this just involves
some small gotos (https://xkcd.com/292):

            code          stack
  before:  33020           2976
  after:   33026 (+0.0%)   2976 (+0.0%)

lfs.c

@@ -10520,8 +10520,9 @@ lfs_ssize_t lfsr_file_write(lfs_t *lfs, lfsr_file_t *file,
     // underlying file, this means no updating file pos or file size
     //
     // since we need to test for this, just return early
+    lfs_size_t written = 0;
     if (size == 0) {
-        return 0;
+        goto noop;
     }
 
     // checkpoint the allocator
@@ -10548,7 +10549,6 @@ lfs_ssize_t lfsr_file_write(lfs_t *lfs, lfsr_file_t *file,
     }
 
     const uint8_t *buffer_ = buffer;
-    lfs_size_t written = 0;
     while (size > 0) {
         // bypass buffer?
         //
@@ -10631,6 +10631,7 @@ lfs_ssize_t lfsr_file_write(lfs_t *lfs, lfsr_file_t *file,
     // update our pos
     file->pos = pos;
 
+noop:;
     // flush if requested
     //
     // this seems unreachable, but it's possible if we transition from
@@ -10873,7 +10874,7 @@ int lfsr_file_truncate(lfs_t *lfs, lfsr_file_t *file, lfs_off_t size_) {
     // do nothing if our size does not change
     lfs_off_t size = lfsr_file_size_(file);
     if (lfsr_file_size_(file) == size_) {
-        return 0;
+        goto noop;
     }
 
     // checkpoint the allocator
@@ -10942,6 +10943,7 @@ int lfsr_file_truncate(lfs_t *lfs, lfsr_file_t *file, lfs_off_t size_) {
     // mark as unsynced
     file->flags |= LFS_F_UNSYNCED;
 
+noop:;
     // flush if requested
     //
     // this seems unreachable, but it's possible if we transition from
@@ -10978,7 +10980,7 @@ int lfsr_file_fruncate(lfs_t *lfs, lfsr_file_t *file, lfs_off_t size_) {
     // do nothing if our size does not change
     lfs_off_t size = lfsr_file_size_(file);
     if (size == size_) {
-        return 0;
+        goto noop;
     }
 
     // checkpoint the allocator
@@ -11075,6 +11077,7 @@ int lfsr_file_fruncate(lfs_t *lfs, lfsr_file_t *file, lfs_off_t size_) {
     // mark as unsynced
     file->flags |= LFS_F_UNSYNCED;
 
+noop:;
     // flush if requested
     //
     // this seems unreachable, but it's possible if we transition from