[GFS2] Fix crc32 calculation in recovery.c

Commit "[GFS2] split and annotate gfs2_log_head" resulted in an incorrect
checksum calculation for log headers. This patch corrects the
problem without resorting to copying the whole log header as
the previous code used to.

Cc: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Steven Whitehouse <swhiteho@redhat.com>