btrfs: avoid superfluous calls to free_extent_map() in btrfs_encoded_read()

Change the control flow of btrfs_encoded_read() so that it doesn't call
free_extent_map() when we know that this has already been done.

Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: Mark Harmstone <maharmstone@fb.com>
Suggested-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>