kselftest/kublk: include message in _Static_assert for C11 compatibility

kselftest/kublk: include message in _Static_assert for C11 compatibility

Add descriptive message in the _Static_assert to comply with the C11
standard requirement to prevent compiler from throwing out error. The
compiler throws an error when _Static_assert is used without a message as
that is a C23 extension.

[] Testing:
The diff between before and after of running the kselftest test of the
module shows no regression on system with x86 architecture

[] Error log:
~/Desktop/kernel-dev/linux-v1/tools/testing/selftests/ublk$ make LLVM=1 W=1
  CC       kublk
In file included from kublk.c:6:
./kublk.h:220:43: error: '_Static_assert' with no message is a C23 extension [-Werror,-Wc23-extensions]
  220 |         _Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
      |                                                  ^
      |                                                  , ""
1 error generated.
In file included from null.c:3:
./kublk.h:220:43: error: '_Static_assert' with no message is a C23 extension [-Werror,-Wc23-extensions]
  220 |         _Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
      |                                                  ^
      |                                                  , ""
1 error generated.
In file included from file_backed.c:3:
./kublk.h:220:43: error: '_Static_assert' with no message is a C23 extension [-Werror,-Wc23-extensions]
  220 |         _Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
      |                                                  ^
      |                                                  , ""
1 error generated.
In file included from common.c:3:
./kublk.h:220:43: error: '_Static_assert' with no message is a C23 extension [-Werror,-Wc23-extensions]
  220 |         _Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
      |                                                  ^
      |                                                  , ""
1 error generated.
In file included from stripe.c:3:
./kublk.h:220:43: error: '_Static_assert' with no message is a C23 extension [-Werror,-Wc23-extensions]
  220 |         _Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
      |                                                  ^
      |                                                  , ""
1 error generated.
In file included from fault_inject.c:11:
./kublk.h:220:43: error: '_Static_assert' with no message is a C23 extension [-Werror,-Wc23-extensions]
  220 |         _Static_assert(UBLK_MAX_QUEUES_SHIFT <= 7);
      |                                                  ^
      |                                                  , ""
1 error generated.
make: *** [../lib.mk:225: ~/Desktop/kernel-dev/linux-v1/tools/testing/selftests/ublk/kublk] Error 1

Link: https://lore.kernel.org/r/20251215085022.7642-1-clintbgeorge@gmail.com
Signed-off-by: Clint George <clintbgeorge@gmail.com>
Reviewed-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>