btrfs-progs: use openat for process_clone in receive

btrfs-progs: use openat for process_clone in receive

This will fix breakage, when doing chrooted receive with cloned paths
outside main subvolume.

Steps to reproduce:
 $ create subvolume
 $ create file
 $ snapshot to snap1
 $ delete file
 $ snapshot to snap2
 $ reflink file from snap1
 $ snapsthot to snap3

 $ send full snap1 | receive --chroot
 $ send incremental snap2 | receive --chroot
 $ send incremental snap3 | receive --chroot

The last step would fail with:

 Chroot to /mnt/recvdir
 At snapshot snap3
 ERROR: failed to open /recvdir/snap1/file. No such file or directory

Signed-off-by: Emil Karlson <jekarlson@gmail.com>
[added reproducer]
Signed-off-by: David Sterba <dsterba@suse.cz>