journal: use consts instead of 1024 and add helper for journal with 1k blocksize

journal: use consts instead of 1024 and add helper for journal with 1k blocksize

Use EXT2_MIN_BLOCK_SIZE, JFS_MIN_JOURNAL_BLOCKS, SUPERBLOCK_SIZE, and
SUPERBLOCK_OFFSET instead of hardcoded 1024 when it is okay, and also
add a helper ext2fs_journal_sb_start() that will return start of
journal sb with special case for fs with 1k block size.

Signed-off-by: Azat Khuzhin <a3at.mail@gmail.com>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>