Re-enable reading byte swapped NFS_MAGIC dumps.

Fix bug introduced in r98542: previously to this revision the byte-swapped
value was compared at this place. The current check is in a conditional
section where the non-byte-swapped value was already checked to be not
the value which is checked again. As byte-swapping is activated afterwards,
it only makes sense if the byte-swapped value is checked.

Submitted by: Keith White <kwhite@site.uottawa.ca>
PR: 200059
MFC after: 1 month
Sponsored by: Essen Hackathon