Renamed lfs->cfg->shrub_size -> lfs->cfg->inline_size

While I think shrub_size is probably the more correct name at a
technical level, inline_size is probably more what users expect and
doesn't require a deeper understanding of filesystem details.

The only risk is that users may think inline_size has no effect on large
files, when in fact it still controls how much of the btree root can be
inlined.

There's also the point that sticking with inline_size maintains
compatibility with both the upstream version and any future version that
has other file representations.

May revisit this, but renaming to lfs->cfg->inline_size for now.

lfs.h

@@ -438,7 +438,7 @@ struct lfs_config {
     // blocksize/4.
     //
     // 0 disables shrubs.
-    lfs_size_t shrub_size;
+    lfs_size_t inline_size;
 
     // Maximum size of a non-block B-tree leaf in bytes. Smaller values may
     // make small random-writes cheaper, but increase metadata overhead. Must