btrfs: make caching_thread use btrfs_find_next_key

btrfs: make caching_thread use btrfs_find_next_key

extent-tree.c has a find_next_key that just walks up the path to find
the next key, but it is used for both the caching stuff and the snapshot
delete stuff.  The snapshot deletion stuff is special so it can't really
use btrfs_find_next_key, but the caching thread stuff can.  We just need
to fix btrfs_find_next_key to deal with ->skip_locking and then it works
exactly the same as the private find_next_key helper.

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