Btrfs: Fix leaf reference cache miss

Due to the optimization for truncate, tree leaves only containing
checksum items can be deleted without being COW'ed first. This causes
reference cache misses. The way to fix the miss is create cache
entries for tree leaves only contain checksum.

This patch also fixes a -EEXIST issue in shared reference cache.

Signed-off-by: Yan Zheng <zheng.yan@oracle.com>