md-bitmap: refactor md_bitmap_init_from_disk

Split the confusing loop in md_bitmap_init_from_disk that iterates over
all chunks but also needs to read and map the pages into three separate
loops: one that iterates over the pages to read them, a second optional
one to iterate over the pages to mark them invalid if the bitmaps are
out of date, and a final one that actually iterates over the chunks.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202306160552.smw0qbmb-lkp@intel.com/


Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Song Liu <song@kernel.org>
Link: https://lore.kernel.org/r/20230615064840.629492-7-hch@lst.de