nilfs2: move NILFS_SUPER_MAGIC to linux/magic.h

move NILFS_SUPER_MAGIC macro to linux/magic.h from linux/nilfs2_fs.h
in the same manner as other filesystem magic number defined in the file.

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