fs: allow cross-vfsmount reflink/dedupe

Currently we disallow reflink and dedupe if the two files aren't on the
same vfsmount.  However we really only need to disallow it if they're
not on the same super block.  It is very common for btrfs to have a main
subvolume that is mounted and then different subvolumes mounted at
different locations.  It's allowed to reflink between these volumes, but
the vfsmount check disallows this.  Instead fix dedupe to check for the
same superblock, and simply remove the vfsmount check for reflink as it
already does the superblock check.

Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>