softmmu: Tidy dirtylimit_dirty_ring_full_time

softmmu: Tidy dirtylimit_dirty_ring_full_time

Drop inline marker: let compiler decide.

Change return type to uint64_t: this matches the computation in the
return statement and the local variable assignment in the caller.

Rename local to dirty_ring_size_MB to fix typo.
Simplify conversion to MiB via qemu_target_page_bits and right shift.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>