Add back-up implementation of be32toh()

Older systems may not have the be32toh function defined. Check for this
and fall back to checking the endianness and calling bswap_32 directly
if needed.  This works on both old and new systems.

[Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>:
address comments raised by Lucas De Marchi [1], update commit message]
[1] http://www.spinics.net/lists/linux-modules/msg01129.html