This is a cleaned-up version of a mremap() fix for "move_one_page()"
by Rajesh Venkatasubramanian <vrajesh@umich.edu>. We could use a NULL
"src" pointer.

Because while we do hold the MM semaphore over the whole sequence, the
destination page table allocation will possibly drop the page table
spinlock.  That in turn can cause a clean source page to be stolen by
page reclaim, causing the source-side "get_one_pte_map_nested()" to
return NULL the second time around even if it didn't on the first case.

So we just check "src" again, and get rid of the bogus TLB invalidate
while we're at it.