libgfs2: Return the inode from build_inum()
This allows the caller to call inode_put() when it's convenient and also
allows the debug message printing to be moved out of the function.
fsck.gfs2 passes the function by reference so it needs a shim until the
other builder functions can be given the same signature.
Signed-off-by: Andrew Price <anprice@redhat.com>