uml: fix build when SLOB is enabled

Reintroduce uml_kmalloc for the benefit of UML libc code.  The
previous tactic of declaring __kmalloc so it could be called directly
from the libc side of the house turned out to be getting too intimate
with slab, and it doesn't work with slob.

So, the uml_kmalloc wrapper is back.  It calls kmalloc or whatever
that translates into, and libc code calls it.

kfree is left alone since that still works, leaving a somewhat
inconsistent API.

Signed-off-by: Jeff Dike <jdike@linux.intel.com>
Cc: WANG Cong <xiyou.wangcong@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>