ipc: fix unused variable warning

Commit a0d092fc introduced the following warning:
ipc/msg.c: In function ?msgctl_down?:
ipc/msg.c:415: warning: ?msqid64? may be used uninitialized in this function

The gcc warning in this case is actually bogus, as msqid64 is touched only
iff cmd == IPC_SET, and in such case, copy_msqid_from_user() initializes
it properly.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>