rbd: be picky about osd request status type

The result field in a ceph osd reply header is a signed 32-bit type,
but rbd code often casually uses int to represent it.

The following changes the types of variables that handle this result
value to be "s32" instead of "int" to be completely explicit about
it.  Only at the point we pass that result to __blk_end_request()
does the type get converted to the plain old int defined for that
interface.

There is almost certainly no binary impact of this change, but I
prefer to show the exact size and signedness of the value since we
know it.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Dan Mick <dan.mick@inktank.com>