ufs: fix s_size/s_dsize users

For UFS2 we need 64bit variants; we even store them in uspi, but
use 32bit ones instead.  One wrinkle is in handling of reserved
space - recalculating it every time had been stupid all along, but
now it would become really ugly.  Just calculate it once...

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>