fdesc_lookup(): the condition to use vn_vget_ino() is always true

The ix number for the fdescfs root is 1, while any fd vnode has the ix
value at least 3.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D39207