Teach ext2fs_open2() to honor the EXT2_FLAG_SUPER_ONLY flag

Teach ext2fs_open2() to honor the EXT2_FLAG_SUPER_ONLY flag

Opening the file system with EXT2_FLAG_SUPER_ONLY will leave
fs->group_desc to be NULL and modify "dumpe2fs -h" and tune2fs when it
is emulating e2label to use this flag.  This speeds up "dumpe2fs -h"
and "e2label" when operating on very large file systems.

To allow other libext2fs functions to work without too many surprises,
ext2fs_group_desc() will read in the block group descriptors on
demand.  This allows "dumpe2fs -h" to be able to read the journal
inode, for example.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cray-bug-id: LUS-5777