btrfs: allow btrfs_bio_fits_in_stripe() to accept bio without any page

Function btrfs_bio_fits_in_stripe() now requires a bio with at least one
page added.  Or btrfs_get_chunk_map() will fail with -ENOENT.

But in fact this requirement is not needed at all, as we can just pass
sectorsize for btrfs_get_chunk_map().

This tiny behavior change is important for later subpage refactoring on
submit_extent_page().

As for 64K page size, we can have a page range with pgoff=0 and size=64K.
If the logical bytenr is just 16K before the stripe boundary, we have to
split the page range into two bios.

This means, we must check page range against stripe boundary, even adding
the range to an empty bio.

This tiny refactoring is for the incoming changes, but on its own,
regular sectorsize == PAGE_SIZE is not affected anyway.

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