Btrfs: fix checks in BTRFS_IOC_CLONE_RANGE

1.  The BTRFS_IOC_CLONE and BTRFS_IOC_CLONE_RANGE ioctls should check
whether the donor file is append-only before writing to it.

2.  The BTRFS_IOC_CLONE_RANGE ioctl appears to have an integer
overflow that allows a user to specify an out-of-bounds range to copy
from the source file (if off + len wraps around).  I haven't been able
to successfully exploit this, but I'd imagine that a clever attacker
could use this to read things he shouldn't.  Even if it's not
exploitable, it couldn't hurt to be safe.

Signed-off-by: Dan Rosenberg <dan.j.rosenberg@gmail.com>
cc: stable@kernel.org
Signed-off-by: Chris Mason <chris.mason@oracle.com>