minor improvement to abd_free_pages()

minor improvement to abd_free_pages()

It doesn't need to have a loop to free page in a single scatterlist
entry because it should be single or compound page. The pages can be
freed in one invocation to __free_pages() for both cases.

Reviewed-by: Gvozden Neskovic <neskovic@gmail.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Richard Yao <ryao@gentoo.org>
Signed-off-by: Jinshan Xiong <jinshan.xiong@gmail.com>
Closes #6057