Btrfs-progs: avoid using btrfs internal subvolume path to send

Btrfs-progs: avoid using btrfs internal subvolume path to send

Steps to reproduce:
	# mkfs.btrfs -f /dev/sda
	# mount /dev/sda /mnt
	# btrfs subvolume create /mnt/foo
	# umount /mnt
	# mount -o subvol=foo /dev/sda /mnt
	# btrfs sub snapshot -r /mnt /mnt/snap
	# btrfs send /mnt/snap > /dev/null

We will fail to send '/mnt/snap',this is because btrfs send try to
open '/mnt/snap' by btrfs internal subvolume path 'foo/snap' rather
than relative path based on mounted point, this will return us 'no
such file or directory',this is not right, fix it.

Reported-by: Thomas Scheiblauer <tom@sharkbay.at>
Signed-off-by: Wang Shilong <wangsl.fnst@cn.fujitsu.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <clm@fb.com>