efi/arm/libstub: Pack FDT after populating it

efi/arm/libstub: Pack FDT after populating it

Commit:

  24d7c494ce46 ("efi/arm-stub: Round up FDT allocation to mapping size")

increased the allocation size for the FDT image created by the stub to a
fixed value of 2 MB, to simplify the former code that made several
attempts with increasing values for the size. This is reasonable
given that the allocation is of type EFI_LOADER_DATA, which is released
to the kernel unless it is explicitly memblock_reserve()d by the early
boot code.

However, this allocation size leaked into the 'size' field of the FDT
header metadata, and so the entire allocation remains occupied by the
device tree binary, even if most of it is not used to store device tree
information.

So call fdt_pack() to shrink the FDT data structure to its minimum size
after populating all the fields, so that the remaining memory is no
longer wasted.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: <stable@vger.kernel.org> # v4.12+
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Fixes: 24d7c494ce46 ("efi/arm-stub: Round up FDT allocation to mapping size")
Link: http://lkml.kernel.org/r/20181114175544.12860-4-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>