ext4: correct cluster len and clusters changed accounting in ext4_mb_mark_bb

ext4: correct cluster len and clusters changed accounting in ext4_mb_mark_bb

BugLink: https://bugs.launchpad.net/bugs/1969110
[ Upstream commit a5c0e2fdf7cea535ba03259894dc184e5a4c2800 ]

ext4_mb_mark_bb() currently wrongly calculates cluster len (clen) and
flex_group->free_clusters. This patch fixes that.

Identified based on code review of ext4_mb_mark_bb() function.

Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Link: https://lore.kernel.org/r/a0b035d536bafa88110b74456853774b64c8ac40.1644992609.git.riteshh@linux.ibm.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>
(cherry picked from commit 572d14e6cec4738d9e2878bd2cf8c070e3b64708)
Signed-off-by: Paolo Pisati <paolo.pisati@canonical.com>