powerpc/pseries: Refactor dlpar_add_lmb() code

Re-factor dlpar_lmb_add() routine by moving the validation of the lmb
flags and the acquireing of the DRC to a wrapper around the work to add
the memory to the system. This is done to make handling of errors
during the addition of the memory easier and to facilitate the upcoming
addition of updating the lmb's affinity prior to adding the memory.

Signed-off-by: Nathan Fontenot <nfont@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>