librbd: use assert_exists() to simplify copyup check

librbd: use assert_exists() to simplify copyup check

Previously we would explicitly STAT the object to see if it exists before
sending the write to the OSD.  Instead, send the write optimistically, and
assert that the object already exists.  This avoids an extra round trip in
the optimistic/common case, and makes the existence check in the initial
first-write case more expensive because we send the data payload along.

Signed-off-by: Sage Weil <sage@inktank.com>