rbd: avoid clang -Wuninitialized warning

clang fails to see that rbd_assert(0) ends in an unreachable code
path and warns about a subsequent use of an uninitialized variable
when CONFIG_PROFILE_ANNOTATED_BRANCHES is set:

drivers/block/rbd.c:2402:4: error: variable 'ret' is used uninitialized whenever 'if' condition is false
      [-Werror,-Wsometimes-uninitialized]
                        rbd_assert(0);
                        ^~~~~~~~~~~~~
drivers/block/rbd.c:563:7: note: expanded from macro 'rbd_assert'
                if (unlikely(!(expr))) {                                \
                    ^~~~~~~~~~~~~~~~~
include/linux/compiler.h:48:23: note: expanded from macro 'unlikely'
 #  define unlikely(x)   (__branch_check__(x, 0, __builtin_constant_p(x)))
                        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
drivers/block/rbd.c:2410:6: note: uninitialized use occurs here
        if (ret) {
            ^~~
drivers/block/rbd.c:2402:4: note: remove the 'if' if its condition is always true
                        rbd_assert(0);
                        ^
drivers/block/rbd.c:563:3: note: expanded from macro 'rbd_assert'
                if (unlikely(!(expr))) {                                \
                ^
drivers/block/rbd.c:2376:9: note: initialize the variable 'ret' to silence this warning
        int ret;
               ^
                = 0
1 error generated.

This seems to be a bug in clang, but is easy to work around by using
an unconditional BUG().

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>