io_uring/kbuf: remove extra ->buf_ring null check

The kernel test robot complains about __io_remove_buffers().

io_uring/kbuf.c:221 __io_remove_buffers() warn: variable dereferenced
before check 'bl->buf_ring' (see line 219)

That check is not needed as ->buf_ring will always be set, so we can
remove it and so silence the warning.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/9a632bbf749d9d911e605255652ce08d18e7d2c6.1681210788.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

io_uring/kbuf.c

@@ -218,14 +218,12 @@ static int __io_remove_buffers(struct io_ring_ctx *ctx,
 	if (bl->is_mapped) {
 		i = bl->buf_ring->tail - bl->head;
 		if (bl->is_mmap) {
-			if (bl->buf_ring) {
 			struct page *page;
 
 			page = virt_to_head_page(bl->buf_ring);
 			if (put_page_testzero(page))
 				free_compound_page(page);
 			bl->buf_ring = NULL;
-			}
 			bl->is_mmap = 0;
 		} else if (bl->buf_nr_pages) {
 			int j;