nilfs2-kmod: use mnt_want_write instead of mnt_want_write_file

This replaces mnt_{want,drop}_write_file() mistakenly backported in
nilfs_ioctl_set_suinfo() function with mnt_{want,drop}_write().

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