btrfs: convert __TRANS_* defines to enum bits

The base transaction bits can be defined as bits in a contiguous
sequence, although right now there's a hole from bit 1 to 8.

The bits are used for btrfs_trans_handle::type, and there's another set
of TRANS_STATE_* defines that are for btrfs_transaction::state. They are
mutually exclusive though the hole in the sequence looks like was made
for the states.

Reviewed-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>