fuseblk: Fix warning in super_setup_bdi_name()

Commit 5f7f7543 "fuse: Convert to separately allocated bdi" didn't
properly handle fuseblk filesystem. When fuse_bdi_init() is called for
that filesystem type, sb->s_bdi is already initialized (by
set_bdev_super()) to point to block device's bdi and consequently
super_setup_bdi_name() complains about this fact when reseting bdi to
the private one.

Fix the problem by properly dropping bdi reference in fuse_bdi_init()
before creating a private bdi in super_setup_bdi_name().

Fixes: 5f7f7543 ("fuse: Convert to separately allocated bdi")
Reported-by: Rakesh Pandit <rakesh@tuxera.com>
Tested-by: Rakesh Pandit <rakesh@tuxera.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>