crypto: cryptd - Fix skcipher instance memory leak

cryptd_skcipher_free() fails to free the struct skcipher_instance
allocated in cryptd_create_skcipher(), leading to a memory leak.  This
is detected by kmemleak on bootup on ARM64 platforms:

 unreferenced object 0xffff80003377b180 (size 1024):
   comm "cryptomgr_probe", pid 822, jiffies 4294894830 (age 52.760s)
   backtrace:
     kmem_cache_alloc_trace+0x270/0x2d0
     cryptd_create+0x990/0x124c
     cryptomgr_probe+0x5c/0x1e8
     kthread+0x258/0x318
     ret_from_fork+0x10/0x1c

Fixes: 4e0958d1 ("crypto: cryptd - Add support for skcipher")
Cc: <stable@vger.kernel.org>
Signed-off-by: Vincent Whitchurch <vincent.whitchurch@axis.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>