[2.1.12] pam: Fix "buffer overflow" in pam ZTS tests on F38

[behlendorf]

Motivation and Context

Resolves an unlikely overflow which is resulting in CI failures on F38.

Description

The pam ZTS tests were reporting a buffer overflow on F38, possibly due to F38 now setting _FORTIFY_SOURCE=3 by default. gdb and valgrind narrowed this down to a snprintf() buffer overflow in zfs_key_config_modify_session_counter(). I'm not clear why this particular snprintf() was being flagged as an overflow, but when I replaced it with an asprintf(), the test passed reliably.

How Has This Been Tested?

See #14842

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.