kfifo: remove support for physically non-contiguous memory

First, there is no such user. The only user of this interface is
caam_rng_fill_async() and that uses kfifo_alloc() -> kmalloc().

Second, the implementation does not allow anything else than direct
mapping and kmalloc() (due to virt_to_phys()), anyway.

Therefore, there is no point in having this dead (and complex) code in
the kernel.

Note the setup_sgl_buf() function now boils down to simple sg_set_buf().
That is called twice from setup_sgl() to take care of kfifo buffer
wrap-around.

setup_sgl_buf() will be extended shortly, so keeping it in place.
Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Stefani Seibold <stefani@seibold.net>
Link: https://lore.kernel.org/r/20240405060826.2521-5-jirislaby@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

lib/kfifo.c

@@ -305,43 +305,15 @@ int __kfifo_to_user(struct __kfifo *fifo, void __user *to,
 }
 EXPORT_SYMBOL(__kfifo_to_user);
 
-static int setup_sgl_buf(struct scatterlist *sgl, void *buf,
-		int nents, unsigned int len)
+static unsigned int setup_sgl_buf(struct scatterlist *sgl, void *buf,
+				  int nents, unsigned int len)
 {
-	int n;
-	unsigned int l;
-	unsigned int off;
-	struct page *page;
-
-	if (!nents)
+	if (!nents || !len)
 		return 0;
 
-	if (!len)
-		return 0;
+	sg_set_buf(sgl, buf, len);
 
-	n = 0;
-	page = virt_to_page(buf);
-	off = offset_in_page(buf);
-	l = 0;
-
-	while (len >= l + PAGE_SIZE - off) {
-		struct page *npage;
-
-		l += PAGE_SIZE;
-		buf += PAGE_SIZE;
-		npage = virt_to_page(buf);
-		if (page_to_phys(page) != page_to_phys(npage) - l) {
-			sg_set_page(sgl, page, l - off, off);
-			sgl = sg_next(sgl);
-			if (++n == nents || sgl == NULL)
-				return n;
-			page = npage;
-			len -= l - off;
-			l = off = 0;
-		}
-	}
-	sg_set_page(sgl, page, len, off);
-	return n + 1;
+	return 1;
 }
 
 static unsigned int setup_sgl(struct __kfifo *fifo, struct scatterlist *sgl,