riscv: Use an atomic xchg in pudp_huge_get_and_clear()

riscv: Use an atomic xchg in pudp_huge_get_and_clear()

Make sure we return the right pud value and not a value that could
have been overwritten in between by a different core.

Fixes: c3cc2a4a3a23 ("riscv: Add support for PUD THP")
Cc: stable@vger.kernel.org
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Link: https://lore.kernel.org/r/20250814-dev-alex-thp_pud_xchg-v1-1-b4704dfae206@rivosinc.com
[pjw@kernel.org: use xchg rather than atomic_long_xchg; avoid atomic op for !CONFIG_SMP like x86]
Signed-off-by: Paul Walmsley <pjw@kernel.org>