fscrypt: split up FS_CRYPTO_BLOCK_SIZE

fscrypt: split up FS_CRYPTO_BLOCK_SIZE

FS_CRYPTO_BLOCK_SIZE is neither the filesystem block size nor the
granularity of encryption.  Rather, it defines two logically separate
constraints that both arise from the block size of the AES cipher:

- The alignment required for the lengths of file contents blocks
- The minimum input/output length for the filenames encryption modes

Since there are way too many things called the "block size", and the
connection with the AES block size is not easily understood, split
FS_CRYPTO_BLOCK_SIZE into two constants FSCRYPT_CONTENTS_ALIGNMENT and
FSCRYPT_FNAME_MIN_MSG_LEN that more clearly describe what they are.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Link: https://lore.kernel.org/r/20220405010914.18519-1-ebiggers@kernel.org