smb/client: fix stale page cache in insert/collapse range

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

smb/client: fix stale page cache in insert/collapse range

smb3_insert_range() and smb3_collapse_range() use
truncate_pagecache_range() to invalidate the affected page cache.
However, if off or old_eof is not page-aligned, the boundary pages are
only partially zeroed and remain uptodate. As a result, the client may
return stale data after a successful insert/collapse range operation.

For example, with 4K pages:

page 0 page 1 page 2
0------4K 4K------8K 8K------12K
^ ^
off=2K old_eof=10K

Page 1 is removed from the page cache, while the boundary pages are
only partially zeroed. After COPYCHUNK moves the data on the server,
these cached pages may still return stale data.

This can be reproduced on a CIFS mount:

bash -c '
FILE=/mnt/scratch/repro

# Use a 6 KiB file so EOF is not page-aligned.
dd if=/dev/urandom of=/tmp/src bs=1K count=6 status=none

# Expected: a 4 KiB hole followed by the original data.
rm -f /tmp/expected
truncate -s 4K /tmp/expected
cat /tmp/src >> /tmp/expected

cp /tmp/src "$FILE"

# Prime the page cache before moving data on the server.
cat "$FILE" > /dev/null

fallocate --insert-range -o 0 -l 4K "$FILE"

if cmp -s /tmp/expected "$FILE"; then
echo "readback: OK"
else
echo "readback: STALE DATA"
fi
'

Fix this by writing back dirty data and discarding the page cache from
the start of the page containing off to EOF before moving data on the
server.