md/bitmap: merge bitmap_file_unmap and bitmap_file_put.

There functions really do one thing together: release the
'bitmap_storage'.  So make them just one function.

Since we removed the locking (previous patch), we don't need to zero
any fields before freeing them, so it all becomes a bit simpler.

Signed-off-by: NeilBrown <neilb@suse.de>