selftests/mm: move write_file helper to vm_util

selftests/mm: move write_file helper to vm_util

thp_settings provides write_file() helper for safely writing to a file and
exit when write failure happens.  It's a very low level helper and many
sub tests need such a helper, not only thp tests.

split_huge_page_test also defines a write_file locally.  The two have
minior differences in return type and used exit api.  And there would be
conflicts if split_huge_page_test wanted to include thp_settings.h because
of different prototype, making it less convenient.

It's possisble to merge the two, although some tests don't use the
kselftest infrastrucutre for testing.  It would also work when using the
ksft_exit_msg() to exit in my test, as the counters are all zero.  Output
will be like:

  TAP version 13
  1..62
  Bail out! /proc/sys/vm/drop_caches1 open failed: No such file or directory
  # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:0 error:0

So here we just keep the version in split_huge_page_test, and move it into
the vm_util.  This makes it easy to maitain and user could just include
one vm_util.h when they don't need thp setting helpers.  Keep the
prototype of void return as the function will exit on any error, return
value is not necessary, and will simply the callers like write_num() and
write_string().

Link: https://lore.kernel.org/20260402014543.1671131-4-chuhu@redhat.com
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Reviewed-by: Lorenzo Stoakes (Oracle) <ljs@kernel.org>
Acked-by: David Hildenbrand (Arm) <david@kernel.org>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Acked-by: Mike Rapoport (Microsoft) <rppt@kernel.org>
Suggested-by: Mike Rapoport <rppt@kernel.org>
Cc: Nico Pache <npache@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>