powerpc/64s/slb: Add no_slb_preload early cmdline param

powerpc/64s/slb: Add no_slb_preload early cmdline param

no_slb_preload cmdline can come useful in quickly disabling and/or
testing the performance impact of userspace slb preloads. Recently there
was a slb multi-hit issue due to slb preload cache which was very
difficult to triage. This cmdline option allows to quickly disable
preloads and verify if the issue exists in preload cache or somewhere
else. This can also be a useful option to see the effect of slb preloads
for any application workload e.g. number of slb faults with or w/o slb
preloads.

with slb_preload:
slb_faults (minimal initrd boot): 15
slb_faults (full systemd boot): 300

with no_slb_preload:
slb_faults (minimal initrd boot): 33
slb_faults (full systemd boot): 138180

Signed-off-by: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/de484b55c45d831bc2db63945f455153c89a9a65.1761834163.git.ritesh.list@gmail.com