sh: Fix up PUD trampling in ranged page table init for X2TLB.

page_table_range_init() presently allocates a PUD page for the 3-level
page table case on X2 TLB configurations on each successive call. This
results in the previous PUD page being trampled when PMDs with an
overlapping PUD are initialized. This case was triggered by putting
persistent kmaps immediately below the fixmap range for highmem.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>