smb/client: invalidate fscache for fallocate range operations

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

smb/client: invalidate fscache for fallocate range operations

smb3_zero_range(), smb3_punch_hole(), smb3_insert_range(), and
smb3_collapse_range() modify file contents through server-side range
operations. These operations discard the affected page cache, but leave
the FS-Cache cookie valid, so a later read may return data cached before
the range operation.

Fix this by invalidating FS-Cache after outstanding I/O has completed
and before modifying the file on the server.

Run the following as root on a CIFS mount with fsc enabled and an active
CacheFiles backend:

bash -c '
MNT=/mnt/cifs
FILE="$MNT/repro"

# Generate four 1 MiB random blocks: [A][B][C][D].
dd if=/dev/urandom of=/tmp/src bs=1M count=4 status=none

# Expected contents after zeroing B: [A][zero][C][D].
cp /tmp/src /tmp/expected
dd if=/dev/zero of=/tmp/expected bs=1M seek=1 count=1 \
conv=notrunc status=none
cp /tmp/src "$FILE"

# Populate FS-Cache, then discard the page cache.
sync
echo 1 > /proc/sys/vm/drop_caches
cat "$FILE" > /dev/null
sync
echo 1 > /proc/sys/vm/drop_caches

fallocate --zero-range -o 1M -l 1M "$FILE"

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

Before this change, the readback differs from /tmp/expected:

readback: STALE DATA

After this change, it matches:

readback: OK