Added lfs_fs_mkconsistent

lfs_fs_mkconsistent allows running the internal consistency operations
(desuperblock/deorphan/demove) on demand and without any other
filesystem changes.

This can be useful for front-loading and persisting consistency operations
when you don't want to pay for this cost on the first write to the
filesystem.

Conveniently, this also offers a way to force the on-disk minor version
to bump, if that is wanted behavior.

Idea from kasper0

lfs.h

@@ -676,6 +676,18 @@ lfs_ssize_t lfs_fs_size(lfs_t *lfs);
 // Returns a negative error code on failure.
 int lfs_fs_traverse(lfs_t *lfs, int (*cb)(void*, lfs_block_t), void *data);
 
+#ifndef LFS_READONLY
+// Attempt to make the filesystem consistent and ready for writing
+//
+// Calling this function is not required, consistency will be implicitly
+// enforced on the first operation that writes to the filesystem, but this
+// function allows the work to be performed earlier and without other
+// filesystem changes.
+//
+// Returns a negative error code on failure.
+int lfs_fs_mkconsistent(lfs_t *lfs);
+#endif
+
 #ifndef LFS_READONLY
 #ifdef LFS_MIGRATE
 // Attempts to migrate a previous version of littlefs