riscv: fix errors in some atomic type aliases

This appears to be a copy-and-paste error that has simply been
overlooked. The tree contains only two calls to any of the affected
variants, but recent additions to the test suite started exercising the
call to atomic_clear_rel_int() in ng_leave_write(), reliably causing
panics.

Apparently, the issue was inherited from the arm64 atomic header. That
instance was addressed in c90baf6817a0, but the fix did not make its way
to RISC-V.

Note that the particular test case ng_macfilter_test:main still appears
to fail on this platform, but this change reduces the panic to a
timeout.

PR: 253237
Reported by: Jenkins, arichardson
Reviewed by: kp, arichardson
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D29064