fs: pass on flags in compat_writev

Fixes: 793b80ef ("vfs: pass a flags argument to vfs_readv/vfs_writev")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Cc: stable@vger.kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>