gfs: Get rid of unnucessary locking in inode_go_dump

Commit 27a2660f ("gfs2: Dump nrpages for inodes and their glocks")
added some locking around reading inode->i_data.nrpages.  That locking
doesn't do anything really, so get rid of it.

With that, the glock argument to ->go_dump() can be made const again as
well.

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