ibcore: Mark write-only variables

Some LinuxKPI lock macros pass need a flags field passed in. This is
written to but never read from so gcc complains.

Fix this by marking the flags variables as unused to quieten the
compiler.

Reviewed by: brooks (earlier version), kib
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D45303

(cherry picked from commit 19782e5bef3403a6ed65d46653f2e70f81dced37)