btrfs: move extent_range_clear_dirty_for_io() into inode.c

The function is only used inside inode.c by compress_file_range(),
so move it to inode.c and unexport it.

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