sparc/mm: Disable preemption in lazy mmu mode

Since commit 38e0edb1 ("mm/apply_to_range: call pte function with
lazy updates") it's been possible for arch_[enter|leave]_lazy_mmu_mode()
to be called without holding a page table lock (for the kernel mappings
case), and therefore it is possible that preemption may occur while in
the lazy mmu mode. The Sparc lazy mmu implementation is not robust to
preemption since it stores the lazy mode state in a per-cpu structure
and does not attempt to manage that state on task switch.

Powerpc had the same issue and fixed it by explicitly disabling
preemption in arch_enter_lazy_mmu_mode() and re-enabling in
arch_leave_lazy_mmu_mode(). See commit b9ef323e ("powerpc/64s:
Disable preemption in hash lazy mmu mode").

Given Sparc's lazy mmu mode is based on powerpc's, let's fix it in the
same way here.

Fixes: 38e0edb1 ("mm/apply_to_range: call pte function with lazy updates")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>