Fix a few issues found by scanner

[preichl]

Following issues were found by Red Hat's OpenScanHub:

fio-3.35/engines/rdma.c:279:3: warning[deadcode.DeadStores]:
Value stored to 'ret' is never read

fio-3.35/server.c:1884:3: warning[deadcode.DeadStores]:
Value stored to 'extended_buf_wp' is never read

fio-3.35/engines/http.c:642: var_tested_neg:
Assigning: "r" = a negative value.
fio-3.35/engines/http.c:714: assign: Assigning: "io_u->error" = "r". fio-3.35/engines/http.c:715: negative_returns: "io_u->error" is passed
to a parameter that cannot be negative.

I suggest changing the data type of io_u->error from uint to int since it appears that negative values are being assigned to it.

[axboe]

The changes looks fine, but the commit title:

Fix a few issues found by scanner

is pretty bad as it means absolutely nothing. What scanner? What issues? I strongly suspect this should be 3 separate commits, not one bundled up.

[preichl]

Thank you for comments, please see 2nd version.

[axboe]

Changes like this HAS to be done very carefully, and not just because some static checker has a blurb about it. The error field is positive values, did you consider that perhaps it's that one use case that may be broken? That would be worthwhile fixing up.

[axboe]

That io_u->error change is highly suspect imho. The other 3 patches look fine. I'd suggest you drop that one for now and we can get the other 3 applied, and then take a deeper look at if this is an inconsistent use of the error field in http instead.

[axboe]

Pulled with that dropped, and then fixed up the error in http.