mm: remove guard around pgd_offset_k() macro

mm: remove guard around pgd_offset_k() macro

The last architecture redefining pgd_offset_k() was IA64 and it was
removed by commit cf8e8658100d ("arch: Remove Itanium (IA-64)
architecture")

There is no need anymore to guard generic version of pgd_offset_k()
with #ifndef pgd_offset_k

Link: https://lkml.kernel.org/r/59d3f47d5615d18cca1986f269be2fcb3df34556.1710589838.git.christophe.leroy@csgroup.eu
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>