Cygwin: lockf: Fix adding a new lock over multiple locks

Cygwin: lockf: Fix adding a new lock over multiple locks

Previously, adding a new lock by lockf() over multiple existing locks
failed. This is due to a bug that lf_setlock() tries to create a lock
that has already been created. This patch fixes the issue.

Addresses: https://cygwin.com/pipermail/cygwin/2024-October/256528.html
Fixes: a998dd705576 ("* flock.cc: Implement all advisory file locking here.")
Reported-by: Christian Franke <Christian.Franke@t-online.de>
Reviewed-by: Corinna Vinschen <corinna@vinschen.de>
Signed-off-by: Takashi Yano <takashi.yano@nifty.ne.jp>