fsck.gfs2: separate check_data function in check_metatree
    
    Function check_metatree had a loop for checking data blocks.  I broke
    this loop into its own function.  That serves two purposes: First, it
    makes check_metatree smaller and easier to read (it's already big an
    unruly).  Second, it avoids looping unnecessarily when the caller
    has no check_data() function of its own.  Net result is faster code.
    
    rhbz#455300