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.

Link: https://lkml.kernel.org/r/20250814-dev-alex-thp_pud_xchg-v1-1-b4704dfae206@rivosinc.com


Fixes: c3cc2a4a ("riscv: Add support for PUD THP")
Signed-off-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Cc: Andrew Donnellan <ajd@linux.ibm.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>