tests: fix S_IFMT undeclared at statx.c

[jlsalvador]

S_IFMT is declared in sys/stat.h, but we cannot include this header because it redeclares the statx function with different argument types. Therefore, we define S_IFMT ourselves, in the same way as the other definitions.

Closes: #17293

Motivation and Context

OpenZFS 2.3.2 fails to compile with musl libc due to S_IFMT not being defined. This issue did not exist in 2.3.1.

The breaking change was introduced in commit 6503f8c, which added a new test command (tests/zfs-tests/cmd/statx). Unfortunately, on some systems (notably musl), including <sys/stat.h> redefines the statx function with incompatible argument types, making it unusable.

To avoid including <sys/stat.h> while still using S_IFMT, we define S_IFMT manually if it is not already defined.

Description

Define S_IFMT as 0170000 if it is not already defined. This value is consistent across major libc implementations, including musl, glibc, and uClibc.

How Has This Been Tested?

Compiling OpenZFS 2.3.2 with this patch using glibc, musl and uClibc.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Performance enhancement (non-breaking change which improves efficiency)
• Code cleanup (non-breaking change which makes code smaller or more readable)
• Breaking change (fix or feature that would cause existing functionality to change)
• Library ABI change (libzfs, libzfs_core, libnvpair, libuutil and libzfsbootenv)
• Documentation (a change to man pages or other documentation)

Checklist:

• My code follows the OpenZFS code style requirements.
• I have updated the documentation accordingly.
• I have read the contributing document.
• I have added tests to cover my changes.
• I have run the ZFS Test Suite with this change applied.
• All commit messages are properly formatted and contain Signed-off-by.

[robn]

Looks right, just waiting to see what the test robots make of it.