btrfs: initialize 'args' to avoid compiler warning in btrfs_ioctl_get_csums()

btrfs: initialize 'args' to avoid compiler warning in btrfs_ioctl_get_csums()

[COMPILER WARNING]
With GCC 11.5.0 and KASAN enabled on ARM, the following warning is
triggered during compiling:

  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;
       |                                           ^~~~

[POSSIBLE FALSE ALERTS]
This seems to be a false alert from certain GCC versions.

The @args is immediately over-written by copy_from_user(), and there is
no code touching that @args until copy_from_user() finished correctly.

[WORKAROUND]
Initialize 'args' to zero, which suppresses the warning.

Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>