cifs: call pagecache_isize_extended() in cifs_setsize() when extending

cifs_setsize() calls truncate_pagecache() but skips
pagecache_isize_extended() on extension.  truncate_setsize() shows
the correct pattern:

  i_size_write(inode, newsize);
  if (newsize > oldsize)
      pagecache_isize_extended(inode, oldsize, newsize);
  truncate_pagecache(inode, newsize);

pagecache_isize_extended() zeroes the tail of the page straddling old
EOF.  Without it, dirty bytes in that region can be written back to
the server, exposing stale data in the newly extended range.

Cc: stable@vger.kernel.org
Cc: David Howells <dhowells@redhat.com>
Signed-off-by: Frank Sorenson <sorenson@redhat.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Paulo Alcantara <pc@manguebit.org>