tests/zfs-tests/cmd/statx.c: avoid statx() conflicts

[jolivain]

With some libc, such as uclibc >= 1.0.57, the libc defines the statx() function, while also internally including <sys/stat.h> (from <fcntl.h> for example), the compilation results to an error due to conflicting types, with the message:

tests/zfs-tests/cmd/statx.c:58:1: error: conflicting types for 'statx'; have 'int(int,  const char *, int,  unsigned int,  void *)'
   58 | statx(int, const char *, int, unsigned int, void *)
      | ^~~~~
In file included from /TestZfsUclibc/host/aarch64-buildroot-linux-uclibc/sysroot/usr/include/sys/stat.h:381,
                 from ./lib/libspl/include/os/linux/sys/stat.h:30,
                 from /TestZfsUclibc/host/aarch64-buildroot-linux-uclibc/sysroot/usr/include/fcntl.h:37,
                 from tests/zfs-tests/cmd/statx.c:29:
/TestZfsUclibc/host/aarch64-buildroot-linux-uclibc/sysroot/usr/include/bits/statx.h:88:5: note: previous declaration of 'statx' with type 'int(int,  const char * restrict,  int,  unsigned int,  struct statx * restrict)'
   88 | int statx (int __dirfd, const char *__restrict __path, int __flags,
      |     ^~~~~

This is because the zfs statx test inconditionally defines a statx() function with a slightly different prototype [1], compared to Glibc [2] and uClibc [3].

The error does not happen with Glibc because its <fcntl.h> header does not include <sys/stat.h> and the zfs-tests/cmd/statx.c does not include <sys/stat.h> either. So the conflict does not happen.

This commit fixes the issue by only defining the statx() prototype only if the libc was detected not to have a working statx() wrapper, and explicitly include the <sys/stat.h> otherwise.

Note: the issue was found in Linux Buildroot [4], while updating uclibc-ng to version 1.0.57. See: [5] [6]. The issue was also reported in [7].

[1] https://github.com/openzfs/zfs/blob/zfs-2.4.1/tests/zfs-tests/cmd/statx.c#L58
[2] https://sourceware.org/git/?p=glibc.git;a=blob;f=io/bits/statx-generic.h;h=9bb9701dc67eccad968b338aeb15d87378fe65cd;hb=ea37298b65bd67f94c3c2640e91ec5865a5019ad#l66
[3] https://github.com/wbx-github/uclibc-ng/blob/v1.0.57/libc/sysdeps/linux/common/bits/statx.h#L88
[4] https://buildroot.org/
[5] https://lore.kernel.org/buildroot/054ee3804bf26bee3dadf8425e0f723c@free.fr/
[6] https://gitlab.com/jolivain/buildroot/-/jobs/13308829775#L237
[7] #18316

Types of changes

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have read the contributing document.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[behlendorf]

Adding the HAVE_STATX guards here as well seems like the cleanest way to resolve the compiler warning.

  tests/zfs-tests/cmd/statx.c: In function '_statx':
  tests/zfs-tests/cmd/statx.c:69:6: error: the address of 'statx' will always evaluate as 'true' [-Werror=address]
    if (statx)
        ^~~~~
  cc1: all warnings being treated as errors