gfs2: Clarify gfs2_block_map

Add a comment about the logical block size for directories.  Rename
"bsize" in gfs2_block_map to "factor".  Fix a typo in the description of
metaptr1.

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