SLUB: use atomic_long_read for atomic_long variables

SLUB is using atomic_read() for variables declared atomic_long_t.
Switch to atomic_long_read().

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>