e2fsck: Reduce unnecessary I/O when checking backup superblock

e2fsck: Reduce unnecessary I/O when checking backup superblock

E2fsck needs to check to see if the backup superblock differs from the
primary superblock.  Previously it was doing so by calling
ext2fs_open(), which does a lot of unnecessary work, including reading
all of the backup block group descriptors.  Avoid this by reading in
the backup superblock directly.

Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>