fstatat(2): handle non-vnode file descriptors for AT_EMPTY_PATH

[kib]

Summary

Set NIRES_EMPTYPATH earlies, to have use of EMPTYPATH recorded even if we are going to return error. When namei_setup() refused to accept dirfd, which is not vnode type, and indicated by ENOTDIR error return, fall back to kern_fstat(dirfd).

[markj]

When does this scenario arise?

[kib]

In D31530#710882, @markj wrote:

When does this scenario arise?

For instance, fstatat(pipefd, "", &stat, AT_EMPTY_PATH) would do it. I have no idea why an app would want to call fstat(pipefd) this way, but this is what Dmitry' test case does, adopted from some Linux trace.