librbd: remove cmp&write sector size restriction

librbd: remove cmp&write sector size restriction

This patch removes the compare and write max sector size len
restriction. We can allow up to stripe unit size accesses if
the access is aligned properly.
To allow larger size compare and write requests in the
journal we split the buffers like we do for writes now.

Signed-off-by: Jonas Pfefferle <pepperjo@japf.ch>