Fix gfs2 in 4.15.y

Hi Greg,

Andrew Price has observed that mounting gfs2 filesystems is currently broken in the 4.15.y kernels. The reason is that commit 'gfs2: Fixes to "Implement iomap for block_map"' was cherry-picked from mainline in 4.15.5 without adding a commit that that commit depends on. Could you please fix that by adding the following commit to 4.15.y:

e8b43fe0c1 gfs2: Clean up {lookup,fillup}_metapath

In addition, could you please add the following commit to 4.15.y as well:

3b5da96e45 gfs2: Fixes to "Implement iomap for block_map" (2)

Both commits can be cherry-picked cleanly.

Thanks a lot, Andreas