Fixed lfs_fs_size doubling metadata-pairs

This was caused by the previous fix for allocations during
lfs_fs_deorphan in this branch. To catch half-orphans during block
allocations we needed to duplicate all metadata-pairs reported to
lfs_fs_traverse. Unfortunately this causes lfs_fs_size to report 2x the
number of metadata-pairs, which would undoubtably confuse users.

The fix here is inelegantly simple, just do a different traversale for
allocations and size measurements. It reuses the same code but touches
slightly different sets of blocks.

Unfortunately, this causes the public lfs_fs_traverse and lfs_fs_size
functions to split in how they report blocks. This is technically
allowed, since lfs_fs_traverse may report blocks multiple times due to
CoW behavior, however it's undesirable and I'm sure there will be some
confusion.

But I don't have a better solution, so from this point lfs_fs_traverse
will be reporting 2x metadata-blocks and shouldn't be used for finding
the number of available blocks on the filesystem.

tests/test_orphans.toml

@@ -31,13 +31,13 @@ code = '''
     lfs_mount(&lfs, &cfg) => 0;
     lfs_stat(&lfs, "parent/orphan", &info) => LFS_ERR_NOENT;
     lfs_stat(&lfs, "parent/child", &info) => 0;
-    lfs_fs_size(&lfs) => 12;
+    lfs_fs_size(&lfs) => 8;
     lfs_unmount(&lfs) => 0;
 
     lfs_mount(&lfs, &cfg) => 0;
     lfs_stat(&lfs, "parent/orphan", &info) => LFS_ERR_NOENT;
     lfs_stat(&lfs, "parent/child", &info) => 0;
-    lfs_fs_size(&lfs) => 12;
+    lfs_fs_size(&lfs) => 8;
     // this mkdir should both create a dir and deorphan, so size
     // should be unchanged
     lfs_mkdir(&lfs, "parent/otherchild") => 0;