e2fsck: don't try backup superblocks beyond the size of the device

e2fsck: don't try backup superblocks beyond the size of the device

Commit f7ef5f3e356d ("e2fsck: check all sparse_super backups") tries
to limit the number of block groups to search for backup superblocks
based on ctx->num_blocks.  Unfortunately, get_backup_sb() gets called
before ctx->num_blocks is set, so we try all block groups up to 2**32
- 1.  Not only does this waste time trying to read from blocks that
don't exist, it triggers the UBSAN checker when multiplying a very
large number by the block size.

Fix this by using ext2fs_get_Device_size(), and if that isn't
available, arbitrarily cap things so that we search block groups up to
128.

Fixes: f7ef5f3e356d ("e2fsck: check all sparse_super backups")
Signed-off-by: Theodore Ts'o <tytso@mit.edu>