ext2fs: add ext2fs_get_stat_i_blocks() function

ext2fs: add ext2fs_get_stat_i_blocks() function

The function ext2fs_inode_i_blocks() is a bit confusing whether it is
returning the inode's i_blocks value, or whether it is returning the
value ala the stat(2) system call, which returns i_blocks in units of
512 byte sectors.  This caused ext2fs_inode_i_blocks() to be
incorrectly used in fuse2fs and the function quota_compute_usage().

To address this, we add a new function, ext2fs_get_stat_i_blocks()
which is clearly labelled what it is returning, and use it in fuse2fs
and quota_compute_usage().  It's also a bit more convenient to use it
in e2fsck, so use it there too.

Reported-by: Wang Shilong <wangshilong1991@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>