gfs2: Get rid of gfs2_log_header_in

Get rid of gfs2_log_header_in by integrating it into get_log_header.
Clean up the crc32 computations and use the same functions for encoding
and decoding to make things less confusing.  Eliminate lh_hash from
gfs2_log_header_host which is completely useless.

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