ext4: convert to alloc_workqueue()

Convert create_workqueue() to alloc_workqueue().  This is an identity
conversion.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: "Theodore Ts'o" <tytso@mit.edu>
Cc: Andreas Dilger <adilger.kernel@dilger.ca>
Cc: linux-ext4@vger.kernel.org