xprtrdma: Fix uninitialized variable

net/sunrpc/xprtrdma/frwr_ops.c:151:32: warning: variable 'rc' is uninitialized when used here [-Wuninitialized]
          trace_xprtrdma_frwr_alloc(mr, rc);
                                        ^~
  net/sunrpc/xprtrdma/frwr_ops.c:127:8: note: initialize the variable 'rc' to silence this warning
          int rc;
                ^
                 = 0
  1 warning generated.

The tracepoint is intended to record the error returned from
ib_alloc_mr(). In the current code there is no other purpose for
@rc, so simply replace it.

Reported-by: kernel test robot <lkp@intel.com>
Fixes: d8cf39a280c3b0 ('xprtrdma: MR-related memory allocation should be allowed to fail')
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>