The error handling was copy and pasted from memdup_user().  It should be
checking for NULL obviously.

Fixes: abccd00f ('btrfs: Fix 32/64-bit problem with BTRFS_SET_RECEIVED_SUBVOL ioctl')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Chris Mason <clm@fb.com>