Currently btrfs_submit_compressed_write() will check
btrfs_bio_fits_in_stripe() each time a new page is going to be added.
Even if compressed extent is small, we don't really need to do that for
every page.

Align the behavior to extent_io.c, by determining the stripe boundary
when allocating a bio.

Unlike extent_io.c, in compressed.c we don't need to bother things like
different bio flags, thus no need to re-use bio_ctrl.

Here we just manually introduce new local variable, next_stripe_start,
and use that value returned from alloc_compressed_bio() to calculate
the stripe boundary.

Then each time we add some page range into the bio, we check if we
reached the boundary.  And if reached, submit it.

Also, since we have @cur_disk_bytenr to determine whether we're the last
bio, we don't need a explicit last_bio: tag for error handling any more.

And since we use @cur_disk_bytenr to wait, there is no need for
pending_bios, also remove it to save some memory of compressed_bio.
Signed-off-by: NQu Wenruo <wqu@suse.com>
Signed-off-by: NDavid Sterba <dsterba@suse.com>