aio: fix async fsync creds

aio: fix async fsync creds

Avi Kivity reports that on fuse filesystems running in a user namespace
asyncronous fsync fails with EOVERFLOW.

The reason is that f_ops->fsync() is called with the creds of the kthread
performing aio work instead of the creds of the process originally
submitting IOCB_CMD_FSYNC.

Fuse sends the creds of the caller in the request header and it needs to
translate the uid and gid into the server's user namespace.  Since the
kthread is running in init_user_ns, the translation will fail and the
operation returns an error.

It can be argued that fsync doesn't actually need any creds, but just
zeroing out those fields in the header (as with requests that currently
don't take creds) is a backward compatibility risk.

Instead of working around this issue in fuse, solve the core of the problem
by calling the filesystem with the proper creds.

Reported-by: Avi Kivity <avi@scylladb.com>
Tested-by: Giuseppe Scrivano <gscrivan@redhat.com>
Fixes: c9582eb0ff7d ("fuse: Fail all requests with invalid uids or gids")
Cc: stable@vger.kernel.org  # 4.18+
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>