x86/smpboot: Fix INIT delay assignment for extended Intel Families

Some old crusty CPUs need an extra delay that slows down booting. See
the comment above 'init_udelay' for details. Newer CPUs don't need the
delay.

Right now, for Intel, Family 6 and only Family 6 skips the delay. That
leaves out both the Family 15 (Pentium 4s) and brand new Family 18/19
models.

The omission of Family 15 (Pentium 4s) seems like an oversight and 18/19
do not need the delay.

Skip the delay on all Intel processors Family 6 and beyond.

Signed-off-by: Sohil Mehta <sohil.mehta@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20250219184133.816753-11-sohil.mehta@intel.com