bsd-user: Always use mmap_find_vma_aligned() in target_mmap()

Massage target_mmap(): calculate alignment once, then
unconditionally call mmap_find_vma_aligned().

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20250308122842.76377-2-philmd@linaro.org>