Btrfs: fix call to btrfs_search_slot in free space cache

We are setting ins_len to 1 even tho we are just modifying an item that should
be there already.  This may cause the search stuff to split nodes on the way
down needelessly.  Set this to 0 since we aren't inserting anything.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>