ipc/shm.c: check for overflows of shm_tot

ipc/shm.c: check for overflows of shm_tot

shm_tot counts the total number of pages used by shm segments.

If SHMALL is ULONG_MAX (or nearly ULONG_MAX), then the number can
overflow.  Subsequent calls to shmctl(,SHM_INFO,) would return wrong
values for shm_tot.

The patch adds a detection for overflows.

Signed-off-by: Manfred Spraul <[email protected]>
Acked-by: Davidlohr Bueso <[email protected]>
Acked-by: KOSAKI Motohiro <[email protected]>
Acked-by: Michael Kerrisk <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>