fscrypt: replace raw loads of info pointer with helper function

fscrypt: replace raw loads of info pointer with helper function

Add and use a helper function fscrypt_get_inode_info_raw().  It loads an
inode's fscrypt info pointer using a raw dereference, which is
appropriate when the caller knows the key setup already happened.

This eliminates most occurrences of inode::i_crypt_info in the source,
in preparation for replacing that with a filesystem-specific field.

Co-developed-by: Christian Brauner <brauner@kernel.org>
Signed-off-by: Eric Biggers <ebiggers@kernel.org>
Link: https://lore.kernel.org/20250810075706.172910-2-ebiggers@kernel.org
Signed-off-by: Christian Brauner <brauner@kernel.org>