fscrypt: move v1 policy key setup to keysetup_v1.c

In preparation for introducing v2 encryption policies which will find
and derive encryption keys differently from the current v1 encryption
policies, move the v1 policy-specific key setup code from keyinfo.c into
keysetup_v1.c.

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