btrfs: update outdated comment for read_block_for_search()

The comment at the top of read_block_for_search() is very outdated, as it
refers to the blocking versus spinning path locking modes. We no longer
have these two locking modes after we switched the btree locks from custom
code to rw semaphores. So update the comment to stop referring to the
blocking mode and put it more up to date.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>