ZTS: Remove non-standard awk hex numbers usage

[amotin]

FreeBSD recently removed non-standard hex numbers support from awk. Neither it supports -n argument, enabling it in gawk. Instead of depending on those rewrite list_file_blocks() function to handle the hex math in shell instead of awk.

Fixes #11141

How Has This Been Tested?

Called the function manually and saw it returning some numbers instead of constant 4MB offset and zero length.

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.

[amotin]

Looking on remaining failures I was able to reproduce one in rsend/recv_dedup_encrypted_zvol. It seems to be a diff regression in FreeBSD 15. The diff binary from FreeBSD 14 shows no problem, so it seems not a ZFS issue. I've sent email to possibly related developer.

PS: Fixed in FreeBSD by https://cgit.freebsd.org/src/commit/?id=893839b119880d3fe8ab18aba4563af6c80cb875 . I have suspicion that it might fix cli_root/zfs_diff/zfs_diff_mangle also, but I am not sure the test is fully correct there either, since it seems to rely on the assumption that diff won't ever exit early.