On x86/um CONFIG_SMP is never defined. As a result, several macros
match the asm-generic variant exactly. Drop the local definitions and
pull in asm-generic/barrier.h instead.
This is in preparation to refactoring this code area.
Signed-off-by: Michael S. Tsirkin <mst@redhat.com> Acked-by: Arnd Bergmann <arnd@arndb.de> Acked-by: Richard Weinberger <richard@nod.at> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>