mm: slab: provide krealloc_array()

When allocating an array of elements, users should check for
multiplication overflow or preferably use one of the provided helpers
like: kmalloc_array().

There's no krealloc_array() counterpart but there are many users who use
regular krealloc() to reallocate arrays.  Let's provide an actual
krealloc_array() implementation.

While at it: add some documentation regarding krealloc.

Link: https://lkml.kernel.org/r/20201109110654.12547-3-brgl@bgdev.pl
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bp@suse.de>
Cc: Christian Knig <christian.koenig@amd.com>
Cc: Christoph Lameter <cl@linux.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Cc: David Airlie <airlied@linux.ie>
Cc: David Rientjes <rientjes@google.com>
Cc...