GCC: Fixes for gcc 14 on Fedora 40

• Workaround dangling pointer in uu_list.c (#16124)
• Fix calloc() transposed arguments in zpool_vdev_os.c
• Make some temp variables unsigned to prevent triggering a '-Werror=alloc-size-larger-than' error.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Closes #16124
Closes #16125