Fixed issue with cold-write after seek to block boundary

This off-by-one error was caused by a slight difference between the
pos argument to lfs_index_find and lfs_index_extend. When pos is on
a block boundary, lfs_index_extend expects block to point before pos,
as it would when writing a file linearly. But when seeking to that
pos, the lfs_index_find to warm things up just supplies the block it
expects pos to be in.

Fixed the off-by-one error and added a test case for several of these
cold seek+writes.

tests/test_seek.sh

@@ -277,5 +277,33 @@ tests/test.py << TEST
     lfs_unmount(&lfs) => 0;
 TEST
 
+echo "--- Boundary seek and write ---"
+tests/test.py << TEST
+    lfs_mount(&lfs, &cfg) => 0;
+    lfs_file_open(&lfs, &file[0], "hello/kitty42", LFS_O_RDWR) => 0;
+
+    size = strlen("hedgehoghog");
+    const lfs_soff_t offsets[] = {512, 1020, 513, 1021, 511, 1019};
+
+    for (int i = 0; i < sizeof(offsets) / sizeof(offsets[0]); i++) {
+        lfs_soff_t off = offsets[i];
+        memcpy(buffer, "hedgehoghog", size);
+        lfs_file_seek(&lfs, &file[0], off, LFS_SEEK_SET) >= 0 => 1;
+        lfs_file_write(&lfs, &file[0], buffer, size) => size;
+        lfs_file_seek(&lfs, &file[0], off, LFS_SEEK_SET) => off+size;
+        lfs_file_read(&lfs, &file[0], buffer, size) => size;
+        memcmp(buffer, "hedgehoghog", size) => 0;
+
+        lfs_file_seek(&lfs, &file[0], 0, LFS_SEEK_SET) => off+size;
+        lfs_file_read(&lfs, &file[0], buffer, size) => size;
+        memcmp(buffer, "kittycatcat", size) => 0;
+
+        lfs_file_sync(&lfs, &file[0]) => 0;
+    }
+
+    lfs_file_close(&lfs, &file[0]) => 0;
+    lfs_unmount(&lfs) => 0;
+TEST
+
 echo "--- Results ---"
 tests/stats.py