sparc64: fix typo in pgd_clear()

It really has to be pgdp, not pgd.

It just happend to work since all callers have 'pgd' as an argument.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>