GFS2: Fix bug-trap in ail flush code

The assert was being tested under the wrong lock, a
legacy of the original code. Also, if it does trigger,
the resulting information was not always a lot of help.

This moves the patch under the correct lock and also
prints out more useful information in tacking down the
source of the problem.

Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>