nilfs2: add read ahead mode to nilfs_btnode_submit_block

This adds mode argument to nilfs_btnode_submit_block() function and
allows it to issue a read-ahead request.

An optional submit_ptr argument is also added to store the actual
block address for which bio is sent.  submit_ptr is used for a series
of read-ahead requests, and helps to decide if each requested block is
continous to the previous one on disk.

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

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