Quiet compiler warnings for fget_noref and fdget_noref

Summary:
Typecasting both parts of the comparison to u_int quiets compiler
warnings about signed/unsigned comparison and takes care of positive
and negative numbers for the file descriptor in a single comparison.

Obtained from: Juniper Netwowrks, Inc.

Reviewers: mjg

Subscribers: imp

Differential Revision: https://reviews.freebsd.org/D39593