ztest: fix broken random call

[robn]

[Sponsors: Klara, Inc., Wasabi Technology, Inc.]

Description

Bad copypasta in 4d451ba, leading to random stuff being blasted all over stack, destroying the program.

The read() call in random_get_bytes_common() random call would end up returning -1 when it tried to write past the end of the stack (errno was EFAULT, not that we were checking it). The assert on the return immediately showed the extent of the damage:

starting main threads...
ASSERT at lib/libspl/random.c:81:random_get_bytes_common()
VERIFY3S(bytes, >=, 0) failed (-1 >= 0x0)
  PID: 37375     COMM: 2^N^G<81><B5>^L^H^L<8D><F6>^B<F4>!Kܵ٪     <BE><9B>^\<D7>Qf<F0>^G<E2>_<D0>˼<94>^P<C1>X}^X^S<A5>^O<C1><BF>B<B6><E0><EB>w<9C><C3>:*^UJ<FA>^O<E8>X<F3>V=<FC><EA><EB>\<EC><E6><FE><84>^Zs^N\<E8>$<9D>^\m<B6><9C><AC>
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  TID: 37375     NAME: ztest

How Has This Been Tested?

Fairly easily reproduced in a ztest run with -X (raidz expansion test).

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.