powerpc/fadump: improve fadump_reserve_mem()

Some code clean-up like using minimal assignments and updating printk
messages. Also, add an 'error_out' label for handling error cleanup
at one place.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/156821343485.5656.10202857091553646948.stgit@hbathini.in.ibm.com