client: handle traceless replies from make_request()

Modify the generic make_request wrapper to retry lookups or getattrs on
requests when the ptarget pointer is specified but there is no trace in
the reply.

Refactor the _create() method to use this, effectively moving all of the
extra_bl code processing into make_request where it is generically
useful.

Signed-off-by: Sage Weil <sage@inktank.com>
client: pass ptarget from link, create, mkdir, symlink, setattr

Use the new make_request functionality to ensure that we properly handle
the re-lookup/getattr when we get a traceless reply back from the MDS.

Note that we have to do this on setattr() because ll_getattr() expects to
return the updated inode metadata to the caller.

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