rxrpc: Fix missing unlock in rxrpc_do_sendmsg()

One of the error paths in rxrpc_do_sendmsg() doesn't unlock the call mutex
before returning.  Fix it to do this.

Note that this still doesn't get rid of the checker warning:

   ../net/rxrpc/sendmsg.c:617:5: warning: context imbalance in 'rxrpc_do_sendmsg' - wrong count at exit

I think the interplay between the socket lock and the call's user_mutex may
be too complicated for checker to analyse, especially as
rxrpc_new_client_call_for_sendmsg(), which it calls, returns with the
call's user_mutex if successful but unconditionally drops the socket lock.

Fixes: e754eba6 ("rxrpc: Provide a cmsg to specify the amount of Tx data for a call")
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: linux-afs@lists.infradead.org
Signed-off-by: David S. Miller <davem@davemloft.net>