io_statx_prep(): use getname_uflags()

the only thing in flags getname_flags() ever cares about is
LOOKUP_EMPTY; anything else is none of its damn business.

Reviewed-by: Jens Axboe <axboe@kernel.dk>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>