slab.h: remove duplicate kmalloc declaration and fix kernel-doc warnings

Fix kernel-doc warning for duplicate definition of 'kmalloc':

Documentation/DocBook/kernel-api.xml:9483: element refentry: validity error : ID API-kmalloc already defined
<refentry id="API-kmalloc">

Also combine the kernel-doc info from the 2 kmalloc definitions into one
block and remove the "see kcalloc" comment since kmalloc now contains the
@flags info.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Pekka Enberg <penberg@kernel.org>