When Katarina activates death lotus, can people like Morgana or Ryze or Maokai stop her channel?
Short answer: Morgana can stop a Katarina with the stun off of her ultimate but not her snare. Ryze cannot stop a channeled ability. Maokai can use his knock up to stop channeled abilities.
Snares and roots are considered movement modifiers. They cause the target to "halt movement entirely, whilst still allowing auto attacks and abilities that do not require movements." Source (For further reference here is a full list of all the CC types and their effects).
Further information about channeling spells: