linux(4): Add a dedicated fstat() implementation

In between kern_fstat() and translate_fd_major_minor(), another process
having the same filedesc could modify or close fd.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D39763

(cherry picked from commit 994ed958ae0545faa86dd43b6e64bf7b64b9220b)