nilfs2: separate function for creating new btree node block

Adds a separate routine for creating a btree node block.  This is a
preparation to reduce the depth of function calls during submitting
btree node buffer.

Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>

diff --cc fs/nilfs2/btnode.c
Simple merge

diff --cc fs/nilfs2/btnode.h
Simple merge