btrfs: remove redundant free_extent_map in btrfs_submit_compressed_read

em can't be non-NULL after the free_extent_map label.  Also remove
the now pointless clearing of em to NULL after freeing it.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>