powerpc/book3s64/radix/tlb: Determine hugepage flush correctly

With a 64K page size flush with start and end:

  (start, end) = (721f680d0000, 721f680e0000)

results in:

  (hstart, hend) = (721f68200000, 721f68000000)

ie. hstart is above hend, which indicates no huge page flush is
needed.

However the current logic incorrectly sets hflush = true in this case,
because hstart != hend.

That causes us to call __tlbie_va_range() passing hstart/hend, to do a
huge page flush even though we don't need to. __tlbie_va_range() will
skip the actual tlbie operation for start > end. But it will still end
up calling fixup_tlbie_va_range() and doing the TLB fixups in there,
which is harmless but unnecessary work.

Reported-by: Bharata B Rao <bharata@linux.ibm.com>
Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.ibm.com>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Drop else case, hflush is already false, flesh out change log]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200513030616.152288-1-aneesh.kumar@linux.ibm.com