bpf: Enable IRQ after irq_work_raise() completes in unit_alloc()

When doing stress test for qp-trie, bpf_mem_alloc() returned NULL
unexpectedly because all qp-trie operations were initiated from
bpf syscalls and there was still available free memory. bpf_obj_new()
has the same problem as shown by the following selftest.

The failure is due to the preemption. irq_work_raise() will invoke
irq_work_claim() first to mark the irq work as pending and then inovke
__irq_work_queue_local() to raise an IPI. So when the current task
which is invoking irq_work_raise() is preempted by other task,
unit_alloc() may return NULL for preemption task as shown below:

task A         task B

unit_alloc()
  // low_watermark = 32
  // free_cnt = 31 after alloc
  irq_work_raise()
    // mark irq work as IRQ_WORK_PENDING
    irq_work_claim()

	       // task B preempts task A
	       unit_alloc()
	         // free_cnt = 30 after alloc
	         // irq work is already PENDING,
	         // so just return
	         irq_work_raise()
	       // does unit_alloc() 30-times
	       ......
	       unit_alloc()
	         // free_cnt = 0 before alloc
	         return NULL

Fix it by enabling IRQ after irq_work_raise() completes. An alternative
fix is using preempt_{disable|enable}_notrace() pair, but it may have
extra overhead. Another feasible fix is to only disable preemption or
IRQ before invoking irq_work_queue() and enable preemption or IRQ after
the invocation completes, but it can't handle the case when
c->low_watermark is 1.

Signed-off-by: Hou Tao <houtao1@huawei.com>
Link: https://lore.kernel.org/r/20230901111954.1804721-2-houtao@huaweicloud.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>