libceph: lock outside send_queued()

Two of the three callers of the osd client's send_queued() function
already hold the osd client mutex and drop it before the call.

Change send_queued() so it assumes the caller holds the mutex, and
update all callers accordingly.  Rename it __send_queued() to match
the convention used elsewhere in the file with respect to the lock.

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