Add lfs_fs_mkconsistent

[geky]

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.

littlefs/lfs.h

Lines 680 to 688 in 259535e

	// 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);

This also offers a way to force the on-disk minor version to bump, if that is wanted behavior. An on-disk minor version bump is being added in #497, so this is convenient timing.

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Not entirely sure about the name, open to suggestions though I think this is an improvement over lfs_fs_forceconsistency which was really meant to be internal only and may imply abnormal behavior.

Idea from @kasper0
See #604 for more info

[geky]

Going ahead and bringing this in so it makes it into the next minor release