nilfs2: add missing initialization of s_mode

An fmode_t argument is passed to kill_block_super() through s_mode
member of the super_block structure.  This is used to release the
block device with the same mode, however, nilfs does not set s_mode
anywhere.

This modifies nilfs_get_sb function to properly initialize the s_mode
member.

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