atomics: Constify loads: More constification

[olce]

Summary

An occurence of an atomic_load*() prototype in riscv's 'atomic.h' had
been missed.

Also, change the cast for atomic_load_acq_ptr() in 'atomic_san.h' to
avoid a build error when used on constant pointers.

While here, remove some now-superfluous __DECONST() uses prior to
calling some atomic_load*() functions.

Only atomic_load*() in ZFS have not been touched. Such changes are
self-contained and will be proposed upstream.

Fixes: 5e9a82e898d5 ("atomics: Constify loads")

[kib]

I think that the commit must be split into much more fine-grained changes.