btree: memset new nodes to 0
btree: memset new nodes to 0

When making a new node, make sure that all its members are initialized
to zero.

Signed-off-by: Ramkumar Ramachandra <5c355d1d38066ed0c5de420d67c1b5ed0687c4bd@gmail.com>

artagnon/phoenixfs,artagnon/phoenixfs,artagnon/phoenixfs