md/bitmap: remove unnecessary page reference manipulations from md/bitmap code

md/bitmap gets a collection of pages representing the bitmap when it
initialises the bitmap, and puts all the references when discarding the
bitmap.

It also occasionally takes extra references without any good reason, and
sometimes drops them ...  though it doesn't always drop them, which can result
in a memory leak.

This patch removes the unnecessary 'get_page' calls, and the corresponding
'put_page' calls.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Adrian Bunk <bunk@kernel.org>