gfs2: Don't reread inodes unnecessarily

In gfs2_create_inode(), we initialize the inode from scratch and then we
write the result to disk.  Clear the GLF_INSTANTIATE_NEEDED glock flag
to indicate that the inode is up to date.  Otherwise, the next time the
inode glock is acquired, gfs2_instantiate() would reread the inode from
disk, which isn't necessary.

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