atomic_common.h: Fix the volatile qualifier placement in atomic_load_ptr

atomic_common.h: Fix the volatile qualifier placement in atomic_load_ptr

This was broken in r357940 which introduced the __typeof use. We need
the volatile qualifier to be on the pointee not the pointer otherwise it
does nothing. This was found by mhorne in D26498, noticing there was a
problem (a spin loop condition was hoisted for RISC-V boot code) but not
the root cause of it.

Reported by:	mhorne
Reviewed by:	mhorne, mjg
Approved by:	mhorne, mjg
Differential Revision:	https://reviews.freebsd.org/D26500