fs_fill: Increase test loop device size to 1GB

On PPC64 the page size is 64K and this causes trouble on btrfs
filesystems of small size (LTP currently uses 300M), the threads
could compete for a very small number of pages/blocks to actually
write the data. So increase minimal device size to 1G avoid the
corner case.

Link: https://lore.kernel.org/ltp/[email protected]/
Reviewed-by: Petr Vorel <[email protected]>
Reviewed-by: Martin Doucha <[email protected]>
Signed-off-by: Wei Gao <[email protected]>

testcases/kernel/fs/fs_fill/fs_fill.c

@@ -123,6 +123,7 @@ static void cleanup(void)
 static struct tst_test test = {
 	.max_runtime = 60,
 	.needs_root = 1,
+	.dev_min_size = 1024,
 	.mount_device = 1,
 	.mntpoint = MNTPOINT,
 	.all_filesystems = 1,