cmd/ztest: avoid PATH_MAX stack allocation in ztest_get_zdb_bin()

[alex-moch]

Calling realpath(path, buf) can trigger fortified header wrappers that allocate a PATH_MAX-sized temporary buffer on the stack, exceeding the 4 KiB frame limit on some systems. Use the heap-allocating realpath(path, NULL) form instead.

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Motivation and Context

When building on Alpine Linux with fortified headers, the compiler emits a warning:

cmd/ztest.c: In function 'ztest_get_zdb_bin.constprop':
cmd/ztest.c:7168:1: warning: the frame size of 4256 bytes is larger than 4096 bytes [-Wframe-larger-than=]
 7168 | }
      | ^

This occurs because the fortified realpath(path, buf) wrapper allocates a PATH_MAX-sized temporary buffer on the stack (typically 4096 bytes), which when combined with the function's other local variables, exceeds the 4 KiB stack frame limit.

Description

This change avoids the fortified wrapper path by using the heap-allocating realpath(path, NULL) form. The resolved path is copied into the caller-provided buffer and freed immediately.

This preserves the existing behavior while avoiding the large temporary stack allocation and keeping the stack frame size below the configured limit.

How Has This Been Tested?

Built on Alpine Linux where the warning was originally observed. The warning no longer appears after this change.

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)
• Quality assurance (non-breaking change which makes the code more robust against bugs)
• 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.