io_uring: fix error pbuf checking

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

io_uring: fix error pbuf checking

Syz reports a problem, which boils down to NULL vs IS_ERR inconsistent

error handling in io_alloc_pbuf_ring().

KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007]

RIP: 0010:__io_remove_buffers+0xac/0x700 io_uring/kbuf.c:341

Call Trace:

<TASK>

io_put_bl io_uring/kbuf.c:378 [inline]

io_destroy_buffers+0x14e/0x490 io_uring/kbuf.c:392

io_ring_ctx_free+0xa00/0x1070 io_uring/io_uring.c:2613

io_ring_exit_work+0x80f/0x8a0 io_uring/io_uring.c:2844

process_one_work kernel/workqueue.c:3231 [inline]

process_scheduled_works+0xa2c/0x1830 kernel/workqueue.c:3312

worker_thread+0x86d/0xd40 kernel/workqueue.c:3390

kthread+0x2f0/0x390 kernel/kthread.c:389

ret_from_fork+0x4b/0x80 arch/x86/kernel/process.c:147

ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244