libceph: apply_upmap()

Previously, pg_to_raw_osds() didn't filter for existent OSDs because
raw_to_up_osds() would filter for "up" ("up" is predicated on "exists")
and raw_to_up_osds() was called directly after pg_to_raw_osds().  Now,
with apply_upmap() call in there, nonexistent OSDs in pg_to_raw_osds()
output can affect apply_upmap().  Introduce remove_nonexistent_osds()
to deal with that.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>