IPC: use ipc_buildid() directly from ipc_addid()

IPC: use ipc_buildid() directly from ipc_addid()

By continuing to consolidate a little the IPC code, each id can be built
directly in ipc_addid() instead of having it built from each callers of
ipc_addid()

And I also remove shm_addid() in order to have, as much as possible, the
same code for shm/sem/msg.

[[email protected]: coding-style fixes]
Signed-off-by: Pierre Peiffer <[email protected]>
Cc: Nadia Derbey <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>