KEYS: fix "ca_keys=" partial key matching

The call to asymmetric_key_hex_to_key_id() from ca_keys_setup()
silently fails with -ENOMEM.  Instead of dynamically allocating
memory from a __setup function, this patch defines a variable
and calls __asymmetric_key_hex_to_key_id(), a new helper function,
directly.

Changelog:
- for clarification, rename hexlen to asciihexlen in
  asymmetric_key_hex_to_key_id()
- add size argument to __asymmetric_key_hex_to_key_id() - David Howells
- inline __asymmetric_key_hex_to_key_id() - David Howells

Cc: <stable@vger.kernel.org> # 3.18+
Fixes: 46963b7 ("KEYS: Overhaul key identification when searching for asymmetric keys")
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>