fuse: fuse_flush must check mapping->flags for errors

commit 9ebce595f63a407c5cec98f98f9da8459b73740a upstream.

fuse_flush() calls write_inode_now() that triggers writeback, but actual
writeback will happen later, on fuse_sync_writes(). If an error happens,
fuse_writepage_end() will set error bit in mapping->flags. So, we have to
check mapping->flags after fuse_sync_writes().

Signed-off-by: Maxim Patlasov <mpatlasov@virtuozzo.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Fixes: 4d99ff8f ("fuse: Turn writeback cache on")
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>