linux/kmem: remove HAVE_ATOMIC64_T and kmem_alloc_used wrappers

Seems like we haven't set it since the SPL was pulled into the main ZFS
tree. In removing the define, I've taken the 64-bit version (ie the one
that _hasn't_ been running since back then) because it looks like its
closer to the intended width by the way its used.

Since the macros ar eno longer needed as a selector, pull those too.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #17551