block: skd: fix incorrect linux/slab_def.h inclusion

block: skd: fix incorrect linux/slab_def.h inclusion

skd includes slab_def.h to get access to the slab cache object size.
However, including this header breaks when we use SLUB or SLOB instead of
the SLAB allocator, since the structure layout is completely different,
as shown by this warning when we build this driver in one of the invalid
configurations with link-time optimizations enabled:

include/linux/slab.h:715:0: error: type of 'kmem_cache_size' does not match original declaration [-Werror=lto-type-mismatch]
 unsigned int kmem_cache_size(struct kmem_cache *s);

mm/slab_common.c:77:14: note: 'kmem_cache_size' was previously declared here
 unsigned int kmem_cache_size(struct kmem_cache *s)
              ^
mm/slab_common.c:77:14: note: code may be misoptimized unless -fno-strict-aliasing is used
include/linux/slab.h:147:0: error: type of 'kmem_cache_destroy' does not match original declaration [-Werror=lto-type-mismatch]
 void kmem_cache_destroy(struct kmem_cache *);

mm/slab_common.c:858:6: note: 'kmem_cache_destroy' was previously declared here
 void kmem_cache_destroy(struct kmem_cache *s)
      ^
mm/slab_common.c:858:6: note: code may be misoptimized unless -fno-strict-aliasing is used
include/linux/slab.h:140:0: error: type of 'kmem_cache_create' does not match original declaration [-Werror=lto-type-mismatch]
 struct kmem_cache *kmem_cache_create(const char *name, size_t size,

mm/slab_common.c:534:1: note: 'kmem_cache_create' was previously declared here
 kmem_cache_create(const char *name, size_t size, size_t align,
 ^

This removes the header inclusion and instead uses the kmem_cache_size()
interface to get the size in a reliable way.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>