evm: Fix memleak in init_desc

tmp_tfm is allocated, but not freed on subsequent kmalloc failure, which
leads to a memory leak.  Free tmp_tfm.

Fixes: d46eb369 ("evm: crypto hash replaced by shash")
Signed-off-by: Dinghao Liu <dinghao.liu@zju.edu.cn>
[zohar@linux.ibm.com: formatted/reworded patch description]
Signed-off-by: Mimi Zohar <zohar@linux.ibm.com>