block/rnbd: Ensure err is always initialized in process_rdma

block/rnbd: Ensure err is always initialized in process_rdma

Clang warns:

drivers/block/rnbd/rnbd-srv.c:150:6: warning: variable 'err' is used
uninitialized whenever 'if' condition is true
[-Wsometimes-uninitialized]
        if (IS_ERR(bio)) {
            ^~~~~~~~~~~
drivers/block/rnbd/rnbd-srv.c:177:9: note: uninitialized use occurs here
        return err;
               ^~~
drivers/block/rnbd/rnbd-srv.c:150:2: note: remove the 'if' if its
condition is always false
        if (IS_ERR(bio)) {
        ^~~~~~~~~~~~~~~~~~
drivers/block/rnbd/rnbd-srv.c:126:9: note: initialize the variable 'err'
to silence this warning
        int err;
               ^
                = 0
1 warning generated.

err is indeed uninitialized when this statement is taken. Ensure that it
is assigned the error value of bio before jumping to the error handling
label.

Fixes: 735d77d4fd28 ("rnbd: remove rnbd_dev_submit_io")
Reported-by: Brooke Basile <brookebasile@gmail.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Acked-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Link: https://github.com/ClangBuiltLinux/linux/issues/1134
Signed-off-by: Jens Axboe <axboe@kernel.dk>