powerpc/mm: Update the memory limit based on direct mapping restrictions

memory limit value specified by the user are further updated such that
the value is 16MB aligned. This is because hash translation mode use
16MB as direct mapping page size. Make sure we update the global
variable 'memory_limit' with the 16MB aligned value such that all kernel
components will see the new aligned value of the memory limit.

Signed-off-by: Aneesh Kumar K.V (IBM) <aneesh.kumar@kernel.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240403083611.172833-3-aneesh.kumar@kernel.org