mm: Introduce pudp/p4dp/pgdp_get() functions

Instead of directly dereferencing page tables entries, which can cause
issues (see commit 20a004e7b017 ("arm64: mm: Use READ_ONCE/WRITE_ONCE when
accessing page tables"), let's introduce new functions to get the
pud/p4d/pgd entries (the pte and pmd versions already exist).

Note that arm pgd_t is actually an array so pgdp_get() is defined as a
macro to avoid a build error.

Those new functions will be used in subsequent commits by the riscv
architecture.

Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20231213203001.179237-3-alexghiti@rivosinc.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

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