GFS2: Remove duplicate log code

The main part of this patch merges the two functions used to
write metadata and data buffers to the log. Most of the code
is common between the two functions, so this provides a nice
clean up, and makes the code more readable.

The gfs2_get_log_desc() function is also extended to take two more
arguments, and thus avoid having to set the length and data1
fields of this strucuture as a separate operation.

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