SLQB would return ZERO_SIZE_PTR rather than NULL if the requested size is too
large. Debugged by Heiko Carstens. Fix this by checking size edge cases up
front rather than in the slab index calculation.

Additionally, if the size parameter was non-constant and too large, then
the checks may not have been performed at all which could cause corruption.

Next, ARCH_KMALLOC_MINALIGN may not be obeyed if size is non-constant. So
test for KMALLOC_MIN_SIZE in that case.

Finally, if KMALLOC_SHIFT_SLQB_HIGH is larger than 2MB, then kmalloc_index
could silently run off the end of its precomputed table and return a -1
index into the kmalloc slab array, which could result in corruption. Extend
this to allow up to 32MB (to match SLAB), and add a compile-time error in
the case that the table is exceeded (also like SLAB).
Tested-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Nick Piggin <npiggin@suse.de>
Signed-off-by: Pekka Enberg <penberg@cs.helsinki.fi>