9p/rdma: remove useless check in cm_event_handler

the client c is always dereferenced to get the rdma struct, so c has to
be a valid pointer at this point.
Gcc would optimize that away but let's make coverity happy...

Link: http://lkml.kernel.org/r/1536339057-21974-3-git-send-email-asmadeus@codewreck.org
Addresses-Coverity-ID: 102778 ("Dereference before null check")
Signed-off-by: Dominique Martinet <dominique.martinet@cea.fr>