[PATCH liburing v2 1/3] Bring back `CONFIG_HAVE_MEMFD_CREATE` to fix Android build error

[Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>]

This partially reverts commit:

  732bf609b670 ("test/io_uring_register: kill old memfd test")

Bring back `CONFIG_HAVE_MEMFD_CREATE` to resolve Android build failures
caused by:

  93d3a7a70b4a ("examples/zcrx: udmabuf backed areas")

It added a call to `memfd_create()`, which is unavailable on some
Android toolchains, leading to the following build error:

```
  zcrx.c:111:10: error: call to undeclared function 'memfd_create'; ISO C99 and \
  later do not support implicit function declarations \
  [-Wimplicit-function-declaration]
    111 |         memfd = memfd_create("udmabuf-test", MFD_ALLOW_SEALING);
        |                 ^
```

This reversion is a preparation patch for a proper fix by ensuring
`memfd_create()` usage is guarded and portable. It's only a partial
revert because the test itself is not restored.

v1 -> v2:
  - Omit the old memfd test because it's not needed anymore.
    Link: https://lore.kernel.org/io-uring/4bc75566-9cb5-42ec-a6b7-16e04062e0c6@kernel.dk

Co-authored-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org>
---
 configure                | 19 +++++++++++++++++++
 test/io_uring_register.c | 11 +++++++++++
 2 files changed, 30 insertions(+)

diff --git a/configure b/configure
index 552f8ae..3c95214 100755
--- a/configure
+++ b/configure
@@ -379,6 +379,22 @@ if compile_prog "" "" "has_ucontext"; then
 fi
 print_config "has_ucontext" "$has_ucontext"
 
+##########################################
+# check for memfd_create(2)
+has_memfd_create="no"
+cat > $TMPC << EOF
+#include <sys/mman.h>
+int main(int argc, char **argv)
+{
+  int memfd = memfd_create("test", 0);
+  return 0;
+}
+EOF
+if compile_prog "-Werror=implicit-function-declaration" "" "has_memfd_create"; then
+  has_memfd_create="yes"
+fi
+print_config "has_memfd_create" "$has_memfd_create"
+
 ##########################################
 # Check NVME_URING_CMD support
 nvme_uring_cmd="no"
@@ -539,6 +555,9 @@ fi
 if test "$array_bounds" = "yes"; then
   output_sym "CONFIG_HAVE_ARRAY_BOUNDS"
 fi
+if test "$has_memfd_create" = "yes"; then
+  output_sym "CONFIG_HAVE_MEMFD_CREATE"
+fi
 if test "$nvme_uring_cmd" = "yes"; then
   output_sym "CONFIG_HAVE_NVME_URING"
 fi
diff --git a/test/io_uring_register.c b/test/io_uring_register.c
index b53a67d..405a812 100644
--- a/test/io_uring_register.c
+++ b/test/io_uring_register.c
@@ -32,6 +32,17 @@ static int pagesize;
 static rlim_t mlock_limit;
 static int devnull;
 
+#if !defined(CONFIG_HAVE_MEMFD_CREATE)
+#include <sys/syscall.h>
+#include <linux/memfd.h>
+
+int memfd_create(const char *name, unsigned int flags)
+{
+	return (int)syscall(SYS_memfd_create, name, flags);
+}
+#endif
+
+
 static int expect_fail(int fd, unsigned int opcode, void *arg,
 		       unsigned int nr_args, int error, int error2)
 {
-- 
Alviro Iskandar Setiawan