powerpc/vdso: Remove unused \tmp param in __get_datapage()

The \tmp param is not used anymore, remove it.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4b13f897dcccce8ae03c031a4598cf26b32e2f1c.1601197618.git.christophe.leroy@csgroup.eu