ipc/shm: fix shmat() nil address after round-down when remapping

ipc/shm: fix shmat() nil address after round-down when remapping

shmat()'s SHM_REMAP option forbids passing a nil address for; this is in
fact the very first thing we check for.  Andrea reported that for
SHM_RND|SHM_REMAP cases we can end up bypassing the initial addr check,
but we need to check again if the address was rounded down to nil.  As
of this patch, such cases will return -EINVAL.

Link: http://lkml.kernel.org/r/20180503204934.kk63josdu6u53fbd@linux-n805
Signed-off-by: Davidlohr Bueso <[email protected]>
Reported-by: Andrea Arcangeli <[email protected]>
Cc: Joe Lawrence <[email protected]>
Cc: Manfred Spraul <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>