btrfs: check return value of btrfs_start_ioctl_transaction() properly

btrfs_start_ioctl_transaction() returns ERR_PTR(), not NULL.
So, it is necessary to use IS_ERR() to check the return value.

Signed-off-by: Tsutomu Itoh <t-itoh@jp.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>