malloc: Use __get_nprocs on arena_get2 (BZ 30945)

This restore the 2.33 semantic for arena_get2.  It was changed by
11a02b03 to avoid arena_get2 call malloc (back when __get_nproc
was refactored to use an scratch_buffer - 903bc7dc).  The
__get_nproc was refactored over then and now it also avoid to call
malloc.

The 11a02b03 did not take in consideration any performance
implication, which should have been discussed properly.  The
__get_nprocs_sched is still used as a fallback mechanism if procfs
and sysfs is not acessible.

Checked on x86_64-linux-gnu.
Reviewed-by: DJ Delorie <dj@redhat.com>