smb3: fix temporary data corruption in collapse range

In the Linux kernel, the following vulnerability has been resolved:

smb3: fix temporary data corruption in collapse range

collapse range doesn't discard the affected cached region

so can risk temporarily corrupting the file data. This

fixes xfstest generic/031

I also decided to merge a minor cleanup to this into the same patch

(avoiding rereading inode size repeatedly unnecessarily) to make it

clearer.