gfs2: Update file times after grabbing glock

In gfs2_page_mkwrite, grab the inode glock in EX mode before calling
file_update_time: grabbing the lock may result in a call to
gfs2_dinode_in, which will reset the file times to their on-disk state.

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