nilfs2: add missing endian conversion on super block magic number

This adds missing endian conversions in comparision of the magic
number of super blocks.  It was coincidence that prior versions didn't
incur problems; the upper byte of the magic number happened to be
equal to the lower byte.  But, semantically it's wrong to depend on
this.

This won't change anything else nor suffer any compatibility issues.

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