integrity: special fs magic

Discussion on the mailing list questioned the use of these
magic values in userspace, concluding these values are already
exported to userspace via statfs and their correct/incorrect
usage is left up to the userspace application.

  - Move special fs magic number definitions to magic.h
  - Add magic.h include

Signed-off-by: Mimi Zohar <zohar@us.ibm.com>
Reviewed-by: James Morris <jmorris@namei.org>
Signed-off-by: James Morris <jmorris@namei.org>