[PATCH v2][next] fsnotify: Avoid -Wflex-array-member-not-at-end warning

["Gustavo A. R. Silva" <gustavoars@kernel.org>]

Use the `DEFINE_FLEX()` helper for an on-stack definition of a
flexible structure where the size of the flexible-array member
is known at compile-time, and refactor the rest of the code,
accordingly.

So, with these changes, fix the following warning:
fs/notify/fdinfo.c:45:36: warning: structure containing a flexible array member is not at the end of another structure [-Wflex-array-member-not-at-end]

Link: https://github.com/KSPP/linux/issues/202
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
---
Changes in v2:
 - Use DEFINE_FLEX() instead of struct_group_tagged().

v1:
 - Link: https://lore.kernel.org/linux-hardening/ZeeaRuTpuxInH6ZB@neat/

 fs/notify/fdinfo.c | 20 ++++++++------------
 1 file changed, 8 insertions(+), 12 deletions(-)

diff --git a/fs/notify/fdinfo.c b/fs/notify/fdinfo.c
index 5c430736ec12..dec553034027 100644
--- a/fs/notify/fdinfo.c
+++ b/fs/notify/fdinfo.c
@@ -41,29 +41,25 @@ static void show_fdinfo(struct seq_file *m, struct file *f,
 #if defined(CONFIG_EXPORTFS)
 static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
 {
-	struct {
-		struct file_handle handle;
-		u8 pad[MAX_HANDLE_SZ];
-	} f;
+	DEFINE_FLEX(struct file_handle, f, f_handle, handle_bytes, MAX_HANDLE_SZ);
 	int size, ret, i;
 
-	f.handle.handle_bytes = sizeof(f.pad);
-	size = f.handle.handle_bytes >> 2;
+	size = f->handle_bytes >> 2;
 
-	ret = exportfs_encode_fid(inode, (struct fid *)f.handle.f_handle, &size);
+	ret = exportfs_encode_fid(inode, (struct fid *)f->f_handle, &size);
 	if ((ret == FILEID_INVALID) || (ret < 0)) {
 		WARN_ONCE(1, "Can't encode file handler for inotify: %d\n", ret);
 		return;
 	}
 
-	f.handle.handle_type = ret;
-	f.handle.handle_bytes = size * sizeof(u32);
+	f->handle_type = ret;
+	f->handle_bytes = size * sizeof(u32);
 
 	seq_printf(m, "fhandle-bytes:%x fhandle-type:%x f_handle:",
-		   f.handle.handle_bytes, f.handle.handle_type);
+		   f->handle_bytes, f->handle_type);
 
-	for (i = 0; i < f.handle.handle_bytes; i++)
-		seq_printf(m, "%02x", (int)f.handle.f_handle[i]);
+	for (i = 0; i < f->handle_bytes; i++)
+		seq_printf(m, "%02x", (int)f->f_handle[i]);
 }
 #else
 static void show_mark_fhandle(struct seq_file *m, struct inode *inode)
-- 
2.34.1