crimson/os/seastore/btree: make updates of lba leaf nodes ptrs

crimson/os/seastore/btree: make updates of lba leaf nodes ptrs
synchronous with contents updates

Since we need merge content of lba leaf nodes when committing
trim/cleaner transactions, and we rely on the child ptrs to determine
whether to modify mappings of pending leaf nodes. We must make sure
the ptr updates and node content updates are synchronous.

See LBALeafNode::merge_content_to() for detail

Signed-off-by: Xuehan Xu <xuxuehan@qianxin.com>