ufs/ffs: detect endian mismatch between machine and filesystem

Mount on a LE machine a filesystem formatted for BE is not supported
currently. This adds a check for the superblock magic number using
swapped bytes to guess and warn the user that it may be a valid
superblock but endian is incompatible.

MFC after: 2 weeks
Reviewed by: mckusick
Obtained from: mckusick, alfredo
Differential Revision: https://reviews.freebsd.org/D37675