powerpc: Chunk calls to flush_dcache_range in arch_*_memory

powerpc: Chunk calls to flush_dcache_range in arch_*_memory

commit 076265907cf9633bbef861c7c2a1c26a8209f283 upstream.

When presented with large amounts of memory being hotplugged
(in my test case, ~890GB), the call to flush_dcache_range takes
a while (~50 seconds), triggering RCU stalls.

This patch breaks up the call into 1GB chunks, calling
cond_resched() inbetween to allow the scheduler to run.

Fixes: fb5924fddf9e ("powerpc/mm: Flush cache on memory hot(un)plug")
Signed-off-by: Alastair D'Silva <alastair@d-silva.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191104023305.9581-6-alastair@au1.ibm.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>