selftests/mm: fix compiler -Wmaybe-uninitialized warning

selftests/mm: fix compiler -Wmaybe-uninitialized warning

Following build warning comes up for cow test as 'transferred' variable has
not been initialized. Fix the warning via zero init for the variable.

  CC       cow
cow.c: In function `do_test_vmsplice_in_parent':
cow.c:365:61: warning: `transferred' may be used uninitialized [-Wmaybe-uninitialized]
  365 |                 cur = read(fds[0], new + total, transferred - total);
      |                                                 ~~~~~~~~~~~~^~~~~~~
cow.c:296:29: note: `transferred' was declared here
  296 |         ssize_t cur, total, transferred;
      |                             ^~~~~~~~~~~
  CC       compaction_test
  CC       gup_longterm

Link: https://lkml.kernel.org/r/20250409095006.1422620-1-anshuman.khandual@arm.com
Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Anshuman Khandual <anshuman.khandual@arm.com>
Cc: David Hildenbrand <david@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>