powerpc: Fix size of st_nlink on 64bit

commit e57f93cc53b7 (powerpc: get rid of nlink_t uses, switch to
explicitly-sized type) changed the size of st_nlink on ppc64 from
a long to a short, resulting in boot failures.

Signed-off-by: Anton Blanchard <anton@samba.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>