mm/migrate: fix indexing bug (off by one) and avoid out of bound access

mm/migrate: fix indexing bug (off by one) and avoid out of bound access

Index was incremented before last use and thus the second array could
dereference to an invalid address (not mentioning the fact that it did
not properly clear the entry we intended to clear).

Link: http://lkml.kernel.org/r/[email protected]
Fixes: 8315ada7f095bf ("mm/migrate: allow migrate_vma() to alloc new page on empty entry")
Signed-off-by: Mark Hairgrove <[email protected]>
Signed-off-by: Jérôme Glisse <[email protected]>
Cc: Reza Arbab <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>