osd: wait for laggy pgs without osd_lock in handle_osd_map

osd: wait for laggy pgs without osd_lock in handle_osd_map

We can't hold osd_lock while blocking because other objectstore completions
need to take osd_lock (e.g., _committed_osd_maps), and those objectstore
completions need to complete in order to finish_splits.  Move the blocking
to the top before we establish any local state in this stack frame since
both the public and cluster dispatchers may race in handle_osd_map and
we are dropping and retaking osd_lock.

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