powerpc/64s: Fix stf mitigation patching w/strict RWX & hash

The stf entry barrier fallback is unsafe to execute in a semi-patched
state, which can happen when enabling/disabling the mitigation with
strict kernel RWX enabled and using the hash MMU.

See the previous commit for more details.

Fix it by changing the order in which we patch the instructions.

Note the stf barrier fallback is only used on Power6 or earlier.

Fixes: bd573a81

 ("powerpc/mm/64s: Allow STRICT_KERNEL_RWX again")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210513140800.1391706-2-mpe@ellerman.id.au