ext4: avoid dividing by 0 in mb_update_avg_fragment_size() when block bitmap corrupt

ext4: avoid dividing by 0 in mb_update_avg_fragment_size() when block bitmap corrupt

[ Upstream commit 993bf0f4c393b3667830918f9247438a8f6fdb5b ]

Determine if bb_fragments is 0 instead of determining bb_free to eliminate
the risk of dividing by zero when the block bitmap is corrupted.

Signed-off-by: Baokun Li <libaokun1@huawei.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/20240104142040.2835097-6-libaokun1@huawei.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>