Fuse ->release() is otherwise asynchronous for the reason that it can
happen in contexts unrelated to close/munmap.

Inode is already written back from fuse_flush().  Add it to
fuse_vma_close() as well to make sure inode dirtying from mmaps also get
written out before the file is released.

Also add error handling.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>