slab: fix kmalloc_nolock() context check for PREEMPT_RT

In the Linux kernel, the following vulnerability has been resolved:

slab: fix kmalloc_nolock() context check for PREEMPT_RT

On PREEMPT_RT kernels, local_lock becomes a sleeping lock. The current

check in kmalloc_nolock() only verifies we're not in NMI or hard IRQ

context, but misses the case where preemption is disabled.

When a BPF program runs from a tracepoint with preemption disabled

(preempt_count > 0), kmalloc_nolock() proceeds to call

local_lock_irqsave() which attempts to acquire a sleeping lock,

triggering:

BUG: sleeping function called from invalid context

in_atomic(): 1, irqs_disabled(): 0, non_block: 0, pid: 6128

preempt_count: 2, expected: 0

Fix this by checking !preemptible() on PREEMPT_RT, which directly

expresses the constraint that we cannot take a sleeping lock when

preemption is disabled. This encompasses the previous checks for NMI

and hard IRQ contexts while also catching cases where preemption is

disabled.