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>

fs/btrfs/compression.c

@@ -572,7 +572,6 @@ void btrfs_submit_compressed_read(struct bio *bio, int mirror_num)
 	cb->orig_bio = bio;
 
 	free_extent_map(em);
-	em = NULL;
 
 	cb->nr_pages = DIV_ROUND_UP(compressed_len, PAGE_SIZE);
 	cb->compressed_pages = kcalloc(cb->nr_pages, sizeof(struct page *), GFP_NOFS);
@@ -629,7 +628,6 @@ void btrfs_submit_compressed_read(struct bio *bio, int mirror_num)
 	kfree(cb->compressed_pages);
 out_free_bio:
 	bio_put(comp_bio);
-	free_extent_map(em);
 out:
 	btrfs_bio_end_io(btrfs_bio(bio), ret);
 }