jbd2: tidy up revoke cache initialisation and destruction

Make revocation cache destruction safe to call if initialisation fails
partially or entirely.  This allows it to be used to cleanup in the case of
initialisation failure, simplifying the code slightly.

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