Fix style of spl_kmem_cache_create()

Fix indentation of code in ifdef's.
Remove obsolete comment.
Make if/else statements more readable by adding braces.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Tony Hutter <hutter2@llnl.gov>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #8459