btrfs: test creating a large number of snapshots of a received subvolume

btrfs: test creating a large number of snapshots of a received subvolume

Test that we can create a large number of snapshots of a received
subvolume without triggering a transaction abort due to leaf item
overflow. Also check that we are able to delete the snapshots and use
the last one for an incremental send/receive despite an item overflow
when updating the uuid tree to insert a BTRFS_UUID_KEY_RECEIVED_SUBVOL
item.

This exercises a bug fixed by the following kernel patch:

  "btrfs: fix transaction abort when snapshotting received subvolumes"

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Zorro Lang <zlang@kernel.org>