Impact: fix unexpected behaviour when pattern number is out of range

Current implementation provides 4 patterns for memtest. The code doesn't
check whether the memtest parameter value exceeds the maximum pattern number.

Instead the memtest code pretends to test with non-existing patterns, e.g.
when booting with memtest=10 I've observed the following

  ...
  early_memtest: pattern num 10
  0000001000 - 0000006000 pattern 0
  ...
  0000001000 - 0000006000 pattern 1
  ...
  0000001000 - 0000006000 pattern 2
  ...
  0000001000 - 0000006000 pattern 3
  ...
  0000001000 - 0000006000 pattern 4
  ...
  0000001000 - 0000006000 pattern 5
  ...
  0000001000 - 0000006000 pattern 6
  ...
  0000001000 - 0000006000 pattern 7
  ...
  0000001000 - 0000006000 pattern 8
  ...
  0000001000 - 0000006000 pattern 9
  ...

But in fact Linux didn't test anything for patterns > 4 as the default
case in memtest() is to leave the function.

I suggest to use the memtest parameter as the number of tests to be
performed and to re-iterate over all existing patterns.

Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>