btrfs: scrub: merge SCRUB_PAGES_PER_RD_BIO and SCRUB_PAGES_PER_WR_BIO

These two values were introduced in commit ff023aac ("Btrfs: add code
to scrub to copy read data to another disk") as an optimization.

But the truth is, block layer scheduler can do whatever it wants to
merge/split bios to improve performance.

Doing such "optimization" is not really going to affect much, especially
considering how good current block layer optimizations are doing.
Remove such old and immature optimization from our code.

Since we're here, also change BUG_ON()s using these two macros to use
ASSERT()s.

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