btrfs: avoid reusing return variable in nested block in btrfs_lookup_bio_sums

The function btrfs_lookup_bio_sums() and a nested if statement declare
ret respectively as blk_status_t and int.

There is no need to store the return value of
search_file_offset_in_bio() to ret as this is a one-time call.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>