do_sys_name_to_handle(): use kzalloc() to fix kernel-infoleak

do_sys_name_to_handle(): use kzalloc() to fix kernel-infoleak

syzbot identified a kernel information leak vulnerability in
do_sys_name_to_handle() and issued the following report [1].

[1]
"BUG: KMSAN: kernel-infoleak in instrument_copy_to_user include/linux/instrumented.h:114 [inline]
BUG: KMSAN: kernel-infoleak in _copy_to_user+0xbc/0x100 lib/usercopy.c:40
 instrument_copy_to_user include/linux/instrumented.h:114 [inline]
 _copy_to_user+0xbc/0x100 lib/usercopy.c:40
 copy_to_user include/linux/uaccess.h:191 [inline]
 do_sys_name_to_handle fs/fhandle.c:73 [inline]
 __do_sys_name_to_handle_at fs/fhandle.c:112 [inline]
 __se_sys_name_to_handle_at+0x949/0xb10 fs/fhandle.c:94
 __x64_sys_name_to_handle_at+0xe4/0x140 fs/fhandle.c:94
 ...

Uninit was created at:
 slab_post_alloc_hook+0x129/0xa70 mm/slab.h:768
 slab_alloc_node mm/slub.c:3478 [inline]
 __kmem_cache_alloc_node+0x5c9/0x970 mm/slub.c:3517
 __do_kmalloc_node mm/slab_common.c:1006 [inline]
 __kmalloc+0x121/0x3c0 mm/slab_common.c:1020
 kmalloc include/linux/slab.h:604 [inline]
 do_sys_name_to_handle fs/fhandle.c:39 [inline]
 __do_sys_name_to_handle_at fs/fhandle.c:112 [inline]
 __se_sys_name_to_handle_at+0x441/0xb10 fs/fhandle.c:94
 __x64_sys_name_to_handle_at+0xe4/0x140 fs/fhandle.c:94
 ...

Bytes 18-19 of 20 are uninitialized
Memory access of size 20 starts at ffff888128a46380
Data copied to user address 0000000020000240"

Per Chuck Lever's suggestion, use kzalloc() instead of kmalloc() to
solve the problem.

Fixes: 990d6c2d7aee ("vfs: Add name to file handle conversion support")
Suggested-by: Chuck Lever III <chuck.lever@oracle.com>
Reported-and-tested-by: <syzbot+09b349b3066c2e0b1e96@syzkaller.appspotmail.com>
Signed-off-by: Nikita Zhandarovich <n.zhandarovich@fintech.ru>
Link: https://lore.kernel.org/r/20240119153906.4367-1-n.zhandarovich@fintech.ru
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Christian Brauner <brauner@kernel.org>