powerpc/mm: Fix size of allocated PGDIR

Commit 6b0e8279 ("powerpc/e500: switch to 64 bits PGD on 85xx
(32 bits)") increased the size of PGD entries but failed to increase
the PGD directory.

Use the size of pgd_t instead of the size of pointers to calculate
the allocated size.

Reported-by: Guenter Roeck <linux@roeck-us.net>
Fixes: 6b0e8279 ("powerpc/e500: switch to 64 bits PGD on 85xx (32 bits)")
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/1cdaacb391cbd3e0240f0e0faf691202874e9422.1723109462.git.christophe.leroy@csgroup.eu