[GFS2] Fix race in logging code

The log lock is dropped prior to io submittion, but
this exposes a hole in which the log data structures
may be going away due to a truncate.
Store the buffer head in a local pointer prior to
dropping the lock and relay on the buffer_head lock
for consitency on the buffer head.

Signed-Off-By: Russell Cattelan <cattelan@redhat.com>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>