fscrypt: rename fscrypt_master_key to fscrypt_direct_key

In preparation for introducing a filesystem-level keyring which will
contain fscrypt master keys, rename the existing 'struct
fscrypt_master_key' to 'struct fscrypt_direct_key'.  This is the
structure in the existing table of master keys that's maintained to
deduplicate the crypto transforms for v1 DIRECT_KEY policies.

I've chosen to keep this table as-is rather than make it automagically
add/remove the keys to/from the filesystem-level keyring, since that
would add a lot of extra complexity to the filesystem-level keyring.

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