Fix spellings of slab allocator section in init/Kconfig

Fix some of the spelling issues. Fix sentences. Discourage SLOB use
since SLUB can pack objects denser.

Signed-off-by: Christoph Lameter <clameter@sgi.com>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>