JBD2: Use the incompat macro for testing the incompat feature.

JBD2_FEATURE_INCOMPAT_ASYNC_COMMIT needs to be checked with
JBD2_HAS_INCOMPAT_FEATURE

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Signed-off-by: Mingming Cao <cmm@us.ibm.com>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>