e4crypt: fix error handling for KEYCTL_GET_KEYRING_ID

e4crypt: fix error handling for KEYCTL_GET_KEYRING_ID

Due to some interesting behaviour in keyctl (as described in the
comments), we use KEYCTL_GET_KEYRING_ID to translate the special value
of KEY_SPEC_SESSION_KEYRING to a real keyring id. However, how we
currently do this is flawed in two ways.

First, if KEYCTL_GET_KEYRING_ID fails, we don't detect it as it returns
-1 and zero is used for an error value in get_keyring_id. Second, if the
user specifies "-k @s" the translation never runs and the undesireable
behavior occurs.

These are both fixed by doing the translation outside of get_keyring_id.

Signed-off-by: Joe Richey <joerichey@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>