Don't write changes to the backup superblocks by default

Don't write changes to the backup superblocks by default

This patch changes ext2fs_open() to set EXT2_FLAG_MASTER_SB_ONLY by
default.  This avoids some problems in e2fsck (reported by Jim Garlick)
where a corrupt journal can end up writing the bad superblock to the
backups.  In general, only e2fsck (after the filesystem is clean),
tune2fs, and resize2fs should change the backup superblocks by default.
Most callers of ext2fs_open() should not be touching anything where the
backups should be touched.  So let's change the defaults to avoid
potential problems.

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