selftests: cachestat: Fix warning on declaration under label

selftests: cachestat: Fix warning on declaration under label

Fix warning caused from declaration under a case label. The proper way
is to declare variable at the beginning of the function. The warning
came from running clang using LLVM=1; and is as follows:

-test_cachestat.c:260:3: warning: label followed by a declaration is a C23 extension [-Wc23-extensions]
  260 |                 char *map = mmap(NULL, filesize, PROT_READ | PROT_WRITE,
      |

Link: https://lore.kernel.org/r/20250929115405.25695-2-sidharthseela@gmail.com
Signed-off-by: Sidharth Seela <sidharthseela@gmail.com>
Reviewed-by: SeongJae Park <sj@kernel.org>
Reviewed-by: wang lian <lianux.mm@gmail.com>
Reviewed-by: Dev Jain <dev.jain@arm.com>
Acked-by: Shuah Khan <skhan@linuxfoundation.org>
Acked-by: Nhat Pham <nphamcs@gmail.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>