fuse: fix bdi naming conflict

Fuse allocates a separate bdi for each filesystem, and registers them
in sysfs with "MAJOR:MINOR" of sb->s_dev (st_dev).  This works fine for
anon devices normally used by fuse, but can conflict with an already
registered BDI for "fuseblk" filesystems, where sb->s_dev represents a
real block device.  In particularl this happens if a non-partitioned
device is being mounted.

Fix by registering with a different name for "fuseblk" filesystems.

Thanks to Ioan Ionita for the bug report.

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Reported-by: Ioan Ionita <opslynx@gmail.com>
Tested-by: Ioan Ionita <opslynx@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>