LinuxKPI: Remove the temporary variable fileid from the macro request_module

The variable fileid stores the result from kern_kldload() but never gets
used. Since the third parameter *fileid of kern_kldload() can be NULL,
this unused variable can be safely removed.

No functional change intended.

Reviewed by: emaste, bz, linuxkpi
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D44583