ext4: Replace lock/unlock_super() with an explicit lock for resizing

ext4: Replace lock/unlock_super() with an explicit lock for resizing

Use a separate lock to protect s_groups_count and the other block
group descriptors which get changed via an on-line resize operation,
so we can stop overloading the use of lock_super().

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