gfs2: add a helper function to get_log_header that can be used elsewhere

Move and re-order the error checks and hash/crc computations into another
function __get_log_header() so it can be used in scenarios where buffer_heads
are not being used for the log header.

Signed-off-by: Abhi Das <adas@redhat.com>
Signed-off-by: Bob Peterson <rpeterso@redhat.com>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>