sparc/mm: Avoid calling arch_enter/leave_lazy_mmu() in set_ptes

With commit 1a10a44d ("sparc64: implement the new page table range
API") set_ptes was added to the sparc architecture. The implementation
included calling arch_enter/leave_lazy_mmu() calls.

The patch removes the usage of arch_enter/leave_lazy_mmu() since this
implies nesting of lazy mmu regions which is not supported. Without this
fix, lazy mmu mode is effectively disabled because we exit the mode
after the first set_ptes:

remap_pte_range()
  -> arch_enter_lazy_mmu()
  -> set_ptes()
      -> arch_enter_lazy_mmu()
      -> arch_leave_lazy_mmu()
  -> arch_leave_lazy_mmu()

Powerpc suffered the same problem and fixed it in a corresponding way
with commit 47b8def9 ("powerpc/mm: Avoid calling
arch_enter/leave_lazy_mmu() in set_ptes").

Fixes: 1a10a44d ("sparc64: implement the new page table range API")
Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>