mm, mremap: fix mremap() expanding vma with addr inside vma

Since 6.1 we have noticed random rpm install failures that were tracked to
mremap() returning -ENOMEM and to commit ca3d76b0 ("mm: add merging
after mremap resize").

The problem occurs when mremap() expands a VMA in place, but using an
starting address that's not vma->vm_start, but somewhere in the middle. 
The extension_pgoff calculation introduced by the commit is wrong in that
case, so vma_merge() fails due to pgoffs not being compatible.  Fix the
calculation.

By the way it seems that the situations, where rpm now expands a vma from
the middle, were made possible also due to that commit, thanks to the
improved vma merging.  Yet it should work just fine, except for the buggy
calculation.

Link: https://lkml.kernel.org/r/20221216163227.24648-1-vbabka@suse.cz


Reported-by: Jiri Slaby <jirislaby@kernel.org>
  Link: https://bugzilla.suse.com/show_bug.cgi?id=1206359
Fixes: ca3d76b0 ("mm: add merging after mremap resize")
Signed-off-by: Vlas...