ext4: fix avefreec in find_group_orlov

ext4: fix avefreec in find_group_orlov

The avefreec should be average free clusters instead
of average free blocks, otherwize Orlov's allocator
will not work properly when bigalloc enabled.

Cc: stable@kernel.org
Signed-off-by: Pan Dong <pandong.peter@bytedance.com>
Link: https://lore.kernel.org/r/20210525073656.31594-1-pandong.peter@bytedance.com
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
(cherry picked from commit c89849cc0259f3d33624cc3bd127685c3c0fa25d)
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>