arm64: mm: Only remove nomap flag for initrd

Commit 177e15f0

 ("arm64: add the initrd region to the linear mapping explicitly")
remove all the flags of the memory used by initrd. This is fine since
MEMBLOCK_MIRROR is not used in arm64.

However with mirrored feature introduced to arm64, this will clear the mirrored
flag used by initrd, which will lead to error log printed by
find_zone_movable_pfns_for_nodes() if the lower 4G range has some non-mirrored
memory.

To solve this problem, only MEMBLOCK_NOMAP flag will be removed via
memblock_clear_nomap().

Signed-off-by: Ma Wupeng <mawupeng1@huawei.com>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Link: https://lore.kernel.org/r/20220614092156.1972846-5-mawupeng1@huawei.com
Acked-by: Mike Rapoport <rppt@linux.ibm.com>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>