nilfs2: eliminate nilfs_btnode_get function

This removes the obsolete nilfs_btnode_get() function and makes
nilfs_btree_get_block() directly call nilfs_btnode_submit_block().

This expansion will provide better opportunity for code optimization.

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

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