nilfs2: eliminate inlines to directly read/write inode of metadata files

Removes two inline functions: nilfs_mdt_read_inode_direct() and
nilfs_mdt_write_inode_direct().

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

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

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