dyn_array: don't break compiling for !CONFIG_SMP

Impact: build failure on uniprocessor

When compiling for !CONFIG_SMP, per_cpu_alloc_dyn_array() would fail
to compile, since it uses per_cpu_offset, which is not defined for
uniprocessor builds.

Hence, do not compile per_cpu_alloc_dyn_array() for !CONFIG_SMP.
Attempting to call this function in a uniprocessor configuration would
be simply wrong in the first place.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>