libext2fs: provide APIs to configure fast commit blocks

libext2fs: provide APIs to configure fast commit blocks

This patch adds new libext2fs that allow configuring number of fast
commit blocks in journal superblock. We also add a struct
ext2fs_journal_params which contains number of fast commit blocks and
number of normal journal blocks. With this patch, the preferred way
for configuring number of blocks with and without fast commits is:

struct ext2fs_journal_params params;

ext2fs_get_journal_params(&params, ...);
params.num_journal_blocks = ...;
params.num_fc_blocks = ...;
ext2fs_create_journal_superblock2(..., &params, ...);
         OR
ext2fs_add_journal_inode3(..., &params, ...);

Signed-off-by: Harshad Shirwadkar <harshadshirwadkar@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>