riscv: mm: pmdp_huge_get_and_clear(): avoid atomic ops when !CONFIG_SMP

riscv: mm: pmdp_huge_get_and_clear(): avoid atomic ops when !CONFIG_SMP

When !CONFIG_SMP, there's no need for atomic operations in
pmdp_huge_get_and_clear(), so, similar to what x86 does, let's not use
atomics in this case.  See also commit 546e42c8c6d94 ("riscv: Use an
atomic xchg in pudp_huge_get_and_clear()").

Cc: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Paul Walmsley <pjw@kernel.org>