Btrfs: cleanup transaction starting and fix journal_info usage

We use journal_info to tell if we're in a nested transaction to make sure we
don't commit the transaction within a nested transaction.  We use another
method to see if there are any outstanding ioctl trans handles, so if we're
starting one do not set current->journal_info, since it will screw with other
filesystems.  This patch also cleans up the starting stuff so there aren't any
magic numbers.

Signed-off-by: Josef Bacik <josef@redhat.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>