arm64: mm: Add ISB instruction to set_pgd()

Commit 6a4cbd63c25a ("Revert "arm64: Remove unnecessary ISBs from
set_{pte,pmd,pud}"") reintroduced ISB instructions to some of our
page table setter functions in light of a recent clarification to the
Armv8 architecture. Although 'set_pgd()' isn't currently used to update
a live page table, add the ISB instruction there too for consistency
with the other macros and to provide some future-proofing if we use it
on live tables in the future.

Reported-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>