powerpc/crash: remove unnecessary NULL check before kvfree()

Fix the following coccicheck build warning:

arch/powerpc/kexec/crash.c:488:2-8: WARNING: NULL check before some
freeing functions is not needed.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202404261048.skfV5DDB-lkp@intel.com/


Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240502182040.774759-1-sourabhjain@linux.ibm.com