btrfs: scrub: aggregate small bitmaps into a larger one

Currently we have several small bitmaps inside scrub_stripe:

- extent_sector_bitmap
- error_bitmap
- io_error_bitmap
- csum_error_bitmap
- meta_error_bitmap
- meta_gen_error_bitmap

All those bitmaps are at most 16 bits long, but unsigned long is
either 32 or 64 (more common) bits.

This means we're wasting 1/2 or 3/4 space for each bitmap.

And we can have 128 scrub_stripe for each device, such wasted space adds up
quickly.

Instead of using a single unsigned long for each bitmap, aggregate them
into a larger bitmap, just like what we're doing for subpage support.

This reduces 24 bytes from each scrub_stripe structure on x86_64
systems.

This will need a lot of macros converting direct bitmap/bit operations into
our scrub_stripe specific helpers, but all those helpers are very small
and can be inlined.

So overall the overhead shouldn't be that huge, and we save quite some
memory space.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

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