[BUG] fs/btrfs: KASAN "args" initialization gripe in btrfs_ioctl_get_csums()

[Paul E. McKenney]

Hello!

This should be considered at least as much a bug report as a fix.  Though
this "fix" does make KASAN happy, so there is that.  ;-)

The following compiler diagnostic appears when building with KASAN on ARM:

In file included from ./include/asm-generic/rwonce.h:26,
                 from ./arch/arm64/include/asm/rwonce.h:81,
                 from ./include/linux/compiler.h:369,
                 from ./include/linux/array_size.h:5,
                 from ./include/linux/kernel.h:16,
                 from fs/btrfs/ioctl.c:6:
In function ‘instrument_copy_from_user_before’,
    inlined from ‘_inline_copy_from_user’ at ./include/linux/uaccess.h:184:2,
    inlined from ‘copy_from_user’ at ./include/linux/uaccess.h:222:9,
    inlined from ‘btrfs_ioctl_get_csums.isra’ at fs/btrfs/ioctl.c:5220:6:
./include/linux/kasan-checks.h:38:27: warning: ‘args’ may be used uninitialized [-Wmaybe-uninitialized]
   38 | #define kasan_check_write __kasan_check_write
./include/linux/instrumented.h:146:9: note: in expansion of macro ‘kasan_check_write’
  146 |         kasan_check_write(to, n);
      |         ^~~~~~~~~~~~~~~~~
fs/btrfs/ioctl.c: In function ‘btrfs_ioctl_get_csums.isra’:
./include/linux/kasan-checks.h:20:6: note: by argument 1 of type ‘const volatile void *’ to ‘__kasan_check_write’ declared here
   20 | bool __kasan_check_write(const volatile void *p, unsigned int size);
      |      ^~~~~~~~~~~~~~~~~~~
fs/btrfs/ioctl.c:5201:43: note: ‘args’ declared here
 5201 |         struct btrfs_ioctl_get_csums_args args;
      |                                           ^~~~

Fix (or at least suppress) this by initializing "args" to "{ 0 }".

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index 9d47d16394fc56..1e28f1411e20f8 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -5198,7 +5198,7 @@ static int btrfs_ioctl_get_csums(struct file *file, void __user *argp)
 	struct btrfs_inode *inode = BTRFS_I(vfs_inode);
 	struct btrfs_fs_info *fs_info = inode->root->fs_info;
 	struct btrfs_root *root = inode->root;
-	struct btrfs_ioctl_get_csums_args args;
+	struct btrfs_ioctl_get_csums_args args = { 0 };
 	BTRFS_PATH_AUTO_FREE(path);
 	const u64 ino = btrfs_ino(inode);
 	const u32 csum_size = fs_info->csum_size;