powerpc/mm: Remove extern from function definition

Function huge_ptep_set_access_flags() has the 'extern' keyword in the
function definition and also in the function declaration. This causes a
warning in 'sparse' since the 'extern' storage class should not be used
in the function definition.

	arch/powerpc/mm/pgtable.c:232:12: warning: function 'huge_ptep_set_access_flags' with external linkage has definition

This patch removes the keyword from the definition part. It also removes
the extern keyword from the declaration part, since checkpatch --strict
complains about it.

Suggested-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>