fscrypt-crypt-util: add --block-number option

fscrypt-crypt-util: add --block-number option

Currently fscrypt-crypt-util assumes that the number of the first block
encrypted/decrypted is 0.  I.e., it replicates either contents
encryption from the start of a file, or encryption of a filename.

However, to easily test compression+encryption on f2fs, we need the
ability to specify a different starting block number.

Add a --block-number option which does this.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Daeho Jeong <daeho43@gmail.com>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>