vm: fix vm_pgoff wrap in stack expansion

vm: fix vm_pgoff wrap in stack expansion

Commit 982134ba6261 ("mm: avoid wrapping vm_pgoff in mremap()") fixed
the case of a expanding mapping causing vm_pgoff wrapping when you used
mremap.  But there was another case where we expand mappings hiding in
plain sight: the automatic stack expansion.

This fixes that case too.

This one also found by Robert Święcki, using his nasty system call
fuzzer tool.  Good job.

Reported-and-tested-by: Robert Święcki <[email protected]>
Cc: [email protected]
Signed-off-by: Linus Torvalds <[email protected]>