backup: Don't cast returns from malloc()

backup: Don't cast returns from malloc()

malloc() returns a (void *) on all of our current platforms. So,
don't bother casting the return value before assigning it - it's
unecessary noise.

Change-Id: I6a878d53007e27179c11d8f745b09dacade4c83d
Reviewed-on: http://gerrit.openafs.org/7460
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Derrick Brashear <shadow@dementix.org>