Btrfs: don't look at the extent buffer level 3 times in a row

We have a bit of debugging in btrfs_search_slot to make sure the level of the
cow block is the same as the original block we were cow'ing.  I don't think I've
ever seen this tripped, so kill it.  This saves us 2 kmap's per level in our
search.  Thanks,

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