Fixed lfs_file_truncate issue where internal state may not be flushed

This was caused by the new lfs_file_rawseek optimization that can skip
flushing when calculated file->pos is unchanged combined with an
implicit expectation in lfs_file_truncate that lfs_file_rawseek
unconditionally sets file->pos.

Because of this assumption, lfs_file_truncate could leave file->pos in
an outdated state while changing the internal file metadata. Humorously,
this was always gauranteed to trigger the skip in lfs_file_rawseek when
we try to restore the file->pos, leaving the file->cache used to do the
CTZ skip-list lookup in a potentially bad state.

The easiest fix is to just update file->pos correctly. Note we don't
want to explicitly flush since we can leverage the same noop
optimization if we truncate to the file position. Which I've added a
test for.

tests/test_truncate.toml

@@ -392,3 +392,48 @@ code = '''
 
     lfs_unmount(&lfs) => 0;
 '''
+
+[[case]] # noop truncate
+define.MEDIUMSIZE = [32, 2048]
+code = '''
+    lfs_format(&lfs, &cfg) => 0;
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file, "baldynoop",
+            LFS_O_RDWR | LFS_O_CREAT) => 0;
+
+    strcpy((char*)buffer, "hair");
+    size = strlen((char*)buffer);
+    for (lfs_off_t j = 0; j < MEDIUMSIZE; j += size) {
+        lfs_file_write(&lfs, &file, buffer, size) => size;
+
+        // this truncate should do nothing
+        lfs_file_truncate(&lfs, &file, j+size) => 0;
+    }
+    lfs_file_size(&lfs, &file) => MEDIUMSIZE;
+
+    lfs_file_seek(&lfs, &file, 0, LFS_SEEK_SET) => 0;
+    // should do nothing again
+    lfs_file_truncate(&lfs, &file, MEDIUMSIZE) => 0;
+    lfs_file_size(&lfs, &file) => MEDIUMSIZE;
+
+    for (lfs_off_t j = 0; j < MEDIUMSIZE; j += size) {
+        lfs_file_read(&lfs, &file, buffer, size) => size;
+        memcmp(buffer, "hair", size) => 0;
+    }
+    lfs_file_read(&lfs, &file, buffer, size) => 0;
+
+    lfs_file_close(&lfs, &file) => 0;
+    lfs_unmount(&lfs) => 0;
+
+    // still there after reboot?
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file, "baldynoop", LFS_O_RDWR) => 0;
+    lfs_file_size(&lfs, &file) => MEDIUMSIZE;
+    for (lfs_off_t j = 0; j < MEDIUMSIZE; j += size) {
+        lfs_file_read(&lfs, &file, buffer, size) => size;
+        memcmp(buffer, "hair", size) => 0;
+    }
+    lfs_file_read(&lfs, &file, buffer, size) => 0;
+    lfs_file_close(&lfs, &file) => 0;
+    lfs_unmount(&lfs) => 0;
+'''