btrfs: declare btrfs_ioctl_search_args_v2::buf as __u8

The variable-sized buffer buf in struct btrfs_ioctl_search_args_v2 is
declared as __u64[], but it holds a packed byte stream of search results,
where all offsets into the buffer are in bytes.

Declaring buf as __u64[] makes it easy for user space to write incorrect
pointer arithmetic: adding a byte offset directly to a __u64 pointer
scales the offset by 8, landing at byte position offset*8 instead of
offset.

This recently caused an infinite loop in btrfs-progs: the accessor read
all-zero data from misaddressed items, which fed zeroed search keys back
into the ioctl loop and spun forever. The issue was worked around at the
time by disabling TREE_SEARCH_V2 entirely in btrfs-progs (d73e69824854:
"btrfs-progs: temporarily disable usage of v2 of search tree ioctl").

The kernel side already treats buf as a byte buffer, so change the
declaration to __u8[] to match the actual semantics and prevent similar
misuse in user space. The change is ABI compatible: both the structure size
and alignment are unchanged.

Fixes: cc68a8a5a4 ("btrfs: new ioctl TREE_SEARCH_V2")
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: You-Kai Zheng <ykzheng@synology.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

include/uapi/linux/btrfs.h

@@ -598,7 +598,7 @@ struct btrfs_ioctl_search_args_v2 {
 	__u64 buf_size;		   /* in - size of buffer
 					    * out - on EOVERFLOW: needed size
 					    *       to store item */
-	__u64 buf[];                       /* out - found items */
+	__u8 buf[];                        /* out - found items */
 };
 
 /* With a @src_length of zero, the range from @src_offset->EOF is cloned! */