fuse: don't overflow LLONG_MAX with end offset

Handle the special case of fuse_readpages() wanting to read the last page
of a hugest file possible and overflowing the end offset in the process.

This is basically to unbreak xfstests:generic/525 and prevent filesystems
from doing bad things with an overflowing offset.
Reported-by: Xiao Yang <ice_yangxiao@163.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/file.c

@@ -803,6 +803,10 @@ static int fuse_do_readpage(struct file *file, struct page *page)
 
 	attr_ver = fuse_get_attr_version(fc);
 
+	/* Don't overflow end offset */
+	if (pos + (desc.length - 1) == LLONG_MAX)
+		desc.length--;
+
 	fuse_read_args_fill(&ia, file, pos, desc.length, FUSE_READ);
 	res = fuse_simple_request(fc, &ia.ap.args);
 	if (res < 0)
@@ -888,6 +892,14 @@ static void fuse_send_readpages(struct fuse_io_args *ia, struct file *file)
 	ap->args.out_pages = true;
 	ap->args.page_zeroing = true;
 	ap->args.page_replace = true;
+
+	/* Don't overflow end offset */
+	if (pos + (count - 1) == LLONG_MAX) {
+		count--;
+		ap->descs[ap->num_pages - 1].length--;
+	}
+	WARN_ON((loff_t) (pos + count) < 0);
+
 	fuse_read_args_fill(ia, file, pos, count, FUSE_READ);
 	ia->read.attr_ver = fuse_get_attr_version(fc);
 	if (fc->async_read) {