fscrypt: separate key lookup from key derivation

Refactor the confusingly-named function 'validate_user_key()' into a new
function 'find_and_derive_key()' which first finds the keyring key, then
does the key derivation.  Among other benefits this avoids the strange
behavior we had previously where if key derivation failed for some
reason, then we would fall back to the alternate key prefix.  Now, we'll
only fall back to the alternate key prefix if a valid key isn't found.

This patch also improves the warning messages that are logged when the
keyring key's payload is invalid.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>