powerpc/drmem: Make LMB walk a bit more flexible

Currently, numa & prom are the only users of drmem LMB walk code.
Loading kdump with kexec_file also needs to walk the drmem LMBs to
setup the usable memory ranges for kdump kernel. But there are couple
of issues in using the code as is. One, walk_drmem_lmb() code is built
into the .init section currently, while kexec_file needs it later.
Two, there is no scope to pass data to the callback function for
processing and/or erroring out on certain conditions.

Fix that by, moving drmem LMB walk code out of .init section, adding
scope to pass data to the callback function and bailing out when an
error is encountered in the callback function.

Signed-off-by: Hari Bathini <hbathini@linux.ibm.com>
Tested-by: Pingfan Liu <piliu@redhat.com>
Reviewed-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/159602282727.575379.3979857013827701828.stgit@hbathini