btrfs: introduce btrfs_lookup_match_dir

btrfs_search_slot is called in multiple places in dir-item.c to search
for a dir entry, and then calling btrfs_match_dir_name to return a
btrfs_dir_item.

In order to reduce the number of callers of btrfs_search_slot, create a
common function that looks for the dir key, and if found call
btrfs_match_dir_item_name.

Signed-off-by: Marcos Paulo de Souza <mpdesouza@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>