ksmbd: don't remove dos attribute xattr on O_TRUNC open

When smb client open file in ksmbd share with O_TRUNC, dos attribute
xattr is removed as well as data in file. This cause the FSCTL_SET_SPARSE
request from the client fails because ksmbd can't update the dos attribute
after setting ATTR_SPARSE_FILE. And this patch fix xfstests generic/469
test also.

Signed-off-by: Namjae Jeon <linkinjeon@kernel.org>
Reviewed-by: Hyunchul Lee <hyc.lee@gmail.com>
Signed-off-by: Steve French <stfrench@microsoft.com>