mm/mempolicy: correctly update prev when policy is equal on mbind

The refactoring in commit f4e9e0e6 ("mm/mempolicy: fix use-after-free
of VMA iterator") introduces a subtle bug which arises when attempting to
apply a new NUMA policy across a range of VMAs in mbind_range().

The refactoring passes a **prev pointer to keep track of the previous VMA
in order to reduce duplication, and in all but one case it keeps this
correctly updated.

The bug arises when a VMA within the specified range has an equivalent
policy as determined by mpol_equal() - which unlike other cases, does not
update prev.

This can result in a situation where, later in the iteration, a VMA is
found whose policy does need to change.  At this point, vma_merge() is
invoked with prev pointing to a VMA which is before the previous VMA.

Since vma_merge() discovers the curr VMA by looking for the one
immediately after prev, it will now be in a situation where this VMA is
incorrect and the merge will not proceed correctly.

This is checked...