gfs2: Unstuff before locking page in gfs2_page_mkwrite

In gfs2_page_mkwrite, unstuff inodes before locking the page.  That
way, we won't have to pass in the locked page to gfs2_unstuff_inode,
and gfs2_unstuff_inode can look up and lock the page itself.

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