btrfs: unexport and move extent_invalidate_folio()

The function extent_invalidate_folio() has only a single caller inside
btree_invalidate_folio().

There is no need to export such a function just for a single caller inside
another file.

Unexport extent_invalidate_folio() and move it to disk-io.c.

And since we're moving the code, update the commit to match the current
style, and remove the seemingly stale comment on the extent state
removal, it's better explained by the comment just before
btrfs_unlock_extent().

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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