nilfs2: replace __attribute__((packed)) with __packed

nilfs2: replace __attribute__((packed)) with __packed

This fixes the following checkpatch.pl warning:

  WARNING: __packed is preferred over __attribute__((packed))
  #23: FILE: export.h:23:
  +} __attribute__ ((packed));

Link: http://lkml.kernel.org/r/1461935747-10380-10-git-send-email-konishi.ryusuke@lab.ntt.co.jp
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>