osd/command tell: check pgid at the right time

osd/command tell: check pgid at the right time

Run a command: "ceph tell osd.0 query", and it outputs the following
error message:
  Error EINVAL: unrecognized command! [{"prefix": "query"}]

In fact, the command "query" exists, but it requires one more input
parameter "pgid", therefore the following error information will be
more appropriate:
  Error EINVAL: no pgid specified

In this patch, we check the parameter pgid after recognizing a command
(like "query" or "list_missing"), rather than before it.

Signed-off-by: Javeme <javaloveme@gmail.com>