btrfs: update outdated comment for read_block_for_search()

The comment at the top of read_block_for_search() is very outdated, as it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: NJosef Bacik <josef@toxicpanda.com>
Signed-off-by: NFilipe Manana <fdmanana@suse.com>
Reviewed-by: NDavid Sterba <dsterba@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>

fs/btrfs/ctree.c

@@ -1390,12 +1390,13 @@ static noinline void unlock_up(struct btrfs_path *path, int level,
 }
 
 /*
- * helper function for btrfs_search_slot.  The goal is to find a block
- * in cache without setting the path to blocking.  If we find the block
- * we return zero and the path is unchanged.
+ * Helper function for btrfs_search_slot() and other functions that do a search
+ * on a btree. The goal is to find a tree block in the cache (the radix tree at
+ * fs_info->buffer_radix), but if we can't find it, or it's not up to date, read
+ * its pages from disk.
  *
- * If we can't find the block, we set the path blocking and do some
- * reada.  -EAGAIN is returned and the search must be repeated.
+ * Returns -EAGAIN, with the path unlocked, if the caller needs to repeat the
+ * whole btree search, starting again from the current root node.
  */
 static int
 read_block_for_search(struct btrfs_root *root, struct btrfs_path *p,