UPSTREAM: scs: add CONFIG_MMU dependency for vfree_atomic()

The shadow call stack implementation fails to build without CONFIG_MMU:

  ld.lld: error: undefined symbol: vfree_atomic
  >>> referenced by scs.c
  >>>               kernel/scs.o:(scs_free) in archive vmlinux.a

Bug: 254441685
Link: https://lkml.kernel.org/r/20240122175204.2371009-1-samuel.holland@sifive.com


Fixes: a2abe7cb ("scs: switch to vmapped shadow stacks")
Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Sami Tolvanen <samitolvanen@google.com>
Cc: Will Deacon <will@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
(cherry picked from commit 6f9dc684)
Signed-off-by: Lee Jones <joneslee@google.com>
Change-Id: I906b8ff918a3ab886af248da9362c0bed542e5d1