drbd: Change drbd_request_detach_interruptible's return type to int

Clang warns when an implicit conversion is done between enumerated
types:

drivers/block/drbd/drbd_state.c:708:8: warning: implicit conversion from
enumeration type 'enum drbd_ret_code' to different enumeration type
'enum drbd_state_rv' [-Wenum-conversion]
                rv = ERR_INTR;
                   ~ ^~~~~~~~

drbd_request_detach_interruptible's only call site is in the return
statement of adm_detach, which returns an int. Change the return type of
drbd_request_detach_interruptible to match, silencing Clang's warning.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <natechancellor@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>