CVE-2026-98686: smb: client: discard post-EOF pagecache when extending a file via zero range

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

smb: client: discard post-EOF pagecache when extending a file via zero range

smb3zerorange() removed the pagecache only for the range being zeroed, so when it extends the file the folio straddling the old EOF - which may hold data dirtied past EOF through an mmap - was never dropped and became visible as file content once the file was extended.

Lower the discard boundary to the smaller of the zero-range offset and the old EOF.