rbd: don't bother setting snapid in rbd_do_request()

For some reason, the snapid field of the osd request header is
explicitly set to CEPH_NOSNAP in rbd_do_request().  Just a few lines
later--with no code that would access this field in between--a call
is made to ceph_calc_raw_layout() passing the snapid provided to
rbd_do_request(), which encodes the snapid value it is provided into
that field instead.

In other words, there is no need to fill in CEPH_NOSNAP, and doing
so suggests it might be necessary.  Don't do that any more.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>