riscv: mm: use xchg() on non-atomic_long_t variables, not atomic_long_xchg()

Let's not call atomic_long_xchg() on something that's not an
atomic_long_t, and just use xchg() instead.  Continues the cleanup
from 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>