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

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

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 of the vnode type, and indicated by ENOTDIR error return,
fall back to kern_fstat(dirfd).

Reported by:	dchagin
Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
MFC after:	1 week
Differential revision:	https://reviews.freebsd.org/D31530