fuse: create fuse_dev on /dev/fuse open instead of mount

fuse: create fuse_dev on /dev/fuse open instead of mount

Allocate struct fuse_dev when opening the device.  This means that unlike
before, ->private_data is always set to a valid pointer.

The use of USE_DEV_SYNC_INIT magic pointer for the private_data is now
replaced with a simple bool sync_init member.

If sync INIT is not set, I/O on the device returns error before mount.
Keep this behavior by checking for the ->fc member.  If fud->fc is set, the
mount has succeeded.  Testing this used READ_ONCE(file->private_data) and
smp_mb() to try and provide the necessary semantics.  Switch this to
smp_store_release() and smp_load_acquire().

Setting fud->fc is protected by fuse_mutex, this is unchanged.

Will need this later so the /dev/fuse open file reference is not held
during FSCONFIG_CMD_CREATE.

Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>