powerpc/fadump: use helper functions to reserve/release cpu notes buffer

Use helper functions to simplify memory allocation, pinning down and
freeing the memory used for CPU notes buffer.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821323555.5656.2486038022572739622.stgit@hbathini.in.ibm.com