fs: btrfs: Crossport btrfs_search_slot() from btrfs-progs

This patch copies the core function, btrfs_search_slot(), from
btrfs-progs.

This version has the following functionality removed:
- The ability to COW tree block
  Related code is commented out, and can be enabled in the future.

- The readahead functionality
  This is abused in kernel. Remove it completely.

With the core function in place, btrfs developers should feel at home now.

This also crossports supporting code like btrfs_previous_item() to
ctree.[ch].

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>