The current kernel_ident_mapping_init() creates the identity
mapping always using 2MB page(PMD level), this patch adds the
1GB page(PUD level) support.
The primary advantage would be better TLB coverage/performance,
because we'd utilize 1GB TLBs instead of 2MB ones.
It is also useful for machines
Kexec setups all identity mappings before booting into the new
kernel, and this will cause extra memory consumption for paging
structures which is quite considerable on modern machines with
huge number of memory.
E.g. On one 32TB machine, in kdump case, it could waste around
128MB (around 4MB/TB)
